Introduce 'override' and 'final' keywords following C++ Core Guidelines

[vweevers]

Closes #598 and implements rule C128 of the C++ Core Guidelines:

Virtual functions should specify exactly one of virtual, override, or final

Reason Readability. Detection of mistakes. Writing explicit virtual, override, or final is self-documenting and enables the compiler to catch mismatch of types and/or names between base and derived classes. However, writing more than one of these three is both redundant and a potential source of errors.
It’s simple and clear:

• virtual means exactly and only “this is a new virtual function.”
• override means exactly and only “this is a non-final overrider.”
• final means exactly and only “this is a final overrider.”

If a base class destructor is declared virtual, one should avoid declaring derived class destructors virtual or override. Some code base and tools might insist on override for destructors, but that is not the recommendation of these guidelines.

[peakji]

Since we're talking about readability and code style, I'd suggest to swap line L336 and L337 to stop the compiler from complaining about initialization order:

  CXX(target) Release/obj.target/leveldown/binding.o
../binding.cc:336:7: warning: field 'priorityWork_' will be initialized after field 'pendingCloseWorker_' [-Wreorder]
      priorityWork_(0),
      ^
1 warning generated.