Enable aleth-bootnode to connect to official Ethereum bootnodes

[halfalicious]

Enable aleth-bootnode to connect to the official Ethereum bootnodes. This is now the behavior by default, but it can be disabled via the new "--no-boostrap" flag.

I've included this functionality because I think it's useful for discovery testing purposes. Additionally, it now enables one to run a discovery-only node that can participate on a public network without one having to manually bootstrap it with another Ethereum node.

[codecov-io]

Codecov Report

Merging #5485 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5485      +/-   ##
==========================================
- Coverage   61.87%   61.86%   -0.01%     
==========================================
  Files         345      345              
  Lines       28690    28699       +9     
  Branches     3262     3265       +3     
==========================================
+ Hits        17751    17756       +5     
- Misses       9773     9781       +8     
+ Partials     1166     1162       -4

[halfalicious]

Looks like this test is flaky (I didn't touch any of this code so it's a test issue not a change issue):

Running 1 test case...
Test Case "pingNotSentAfterPongForKnownNode": 
/home/builder/project/libdevcore/concurrent_queue.h(47): fatal error: in "_T dev::concurrent_queue<_T, _QueueT>::pop(const milliseconds&) [with _T = std::vector<unsigned char>; _QueueT = std::queue<std::vector<unsigned char>, std::deque<std::vector<unsigned char>, std::allocator<std::vector<unsigned char> > > >; std::chrono::milliseconds = std::chrono::duration<long int, std::ratio<1l, 1000l> >]": boost::exception_detail::clone_impl<dev::WaitTimeout>: /home/builder/project/libdevcore/concurrent_queue.h(47): Throw in function _T dev::concurrent_queue<_T, _QueueT>::pop(const milliseconds&) [with _T = std::vector<unsigned char>; _QueueT = std::queue<std::vector<unsigned char>, std::deque<std::vector<unsigned char>, std::allocator<std::vector<unsigned char> > > >; std::chrono::milliseconds = std::chrono::duration<long int, std::ratio<1l, 1000l> >]
Dynamic exception type: boost::exception_detail::clone_impl<dev::WaitTimeout>

I'll open a new issue for this

[halfalicious]

Looks like this test is flaky (I didn't touch any of this code so it's a test issue not a change issue):

Running 1 test case...
Test Case "pingNotSentAfterPongForKnownNode": 
/home/builder/project/libdevcore/concurrent_queue.h(47): fatal error: in "_T dev::concurrent_queue<_T, _QueueT>::pop(const milliseconds&) [with _T = std::vector<unsigned char>; _QueueT = std::queue<std::vector<unsigned char>, std::deque<std::vector<unsigned char>, std::allocator<std::vector<unsigned char> > > >; std::chrono::milliseconds = std::chrono::duration<long int, std::ratio<1l, 1000l> >]": boost::exception_detail::clone_impl<dev::WaitTimeout>: /home/builder/project/libdevcore/concurrent_queue.h(47): Throw in function _T dev::concurrent_queue<_T, _QueueT>::pop(const milliseconds&) [with _T = std::vector<unsigned char>; _QueueT = std::queue<std::vector<unsigned char>, std::deque<std::vector<unsigned char>, std::allocator<std::vector<unsigned char> > > >; std::chrono::milliseconds = std::chrono::duration<long int, std::ratio<1l, 1000l> >]
Dynamic exception type: boost::exception_detail::clone_impl<dev::WaitTimeout>

I'll open a new issue for this

Looks like this test is flaky (I didn't touch any of this code so it's a test issue not a change issue):

Running 1 test case...
Test Case "pingNotSentAfterPongForKnownNode": 
/home/builder/project/libdevcore/concurrent_queue.h(47): fatal error: in "_T dev::concurrent_queue<_T, _QueueT>::pop(const milliseconds&) [with _T = std::vector<unsigned char>; _QueueT = std::queue<std::vector<unsigned char>, std::deque<std::vector<unsigned char>, std::allocator<std::vector<unsigned char> > > >; std::chrono::milliseconds = std::chrono::duration<long int, std::ratio<1l, 1000l> >]": boost::exception_detail::clone_impl<dev::WaitTimeout>: /home/builder/project/libdevcore/concurrent_queue.h(47): Throw in function _T dev::concurrent_queue<_T, _QueueT>::pop(const milliseconds&) [with _T = std::vector<unsigned char>; _QueueT = std::queue<std::vector<unsigned char>, std::deque<std::vector<unsigned char>, std::allocator<std::vector<unsigned char> > > >; std::chrono::milliseconds = std::chrono::duration<long int, std::ratio<1l, 1000l> >]
Dynamic exception type: boost::exception_detail::clone_impl<dev::WaitTimeout>

I'll open a new issue for this

Opened #5488

[halfalicious]

Rebased

[chfast]

Actually, does it have to be a map at all? I think an std::array would work.