Cap max incoming new block hashes at c_maxIncomingNewHashes

[halfalicious]

Fix #5677

When we receive a NewBlockHashes ethereum capability packet which contains more hashes than c_maxIncomingNewHashes, process the first c_maxIncomingNewHashes hashes rather than disabling the peer.

This bring's Aleth's behavior in line with Parity's (and I suspect Geth's as well).

Parity's processing of the NewBlockHashes packet is here: https://github.com/paritytech/parity-ethereum/blob/7f707fa524baaaccac499d69f9e452b380c31ffa/ethcore/sync/src/chain/handler.rs#L219-L288

[chfast]

Simple enough I can understand it. @gumb0 ?

[gumb0]

Looks fine, did you see this happening?

(our own code looks to be sending out no more than 20 hashes)

[halfalicious]

@gumb0 Haven't checked to see if we're actually hitting this, came across this issue when comparing our behavior to the Ethereum Wire Protocol spec / other clients' behavior

[halfalicious]

gcc CI failure looks infrastructure-related:

==> Uploading
    .url https://codecov.io
    .query build=12468.0&slug=ethereum%2Faleth&commit=46a0426787f2e86b317820272f08af170b807661&branch=return-maxhashes&yaml=codecov.yml&package=py2.0.15&service=circleci&job=12468.0
    Pinging Codecov...
Error: HTTPSConnectionPool(host='codecov.io', port=443): Max retries exceeded with url: /upload/v4?build=12468.0&slug=ethereum%2Faleth&commit=46a0426787f2e86b317820272f08af170b807661&branch=return-maxhashes&yaml=codecov.yml&package=py2.0.15&service=circleci&job=12468.0 (Caused by NewConnectionError('<requests.packages.urllib3.connection.VerifiedHTTPSConnection object at 0x7f0230dc3978>: Failed to establish a new connection: [Errno 110] Connection timed out',))

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[codecov-io]

Codecov Report

Merging #5676 into master will increase coverage by 0.06%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5676      +/-   ##
==========================================
+ Coverage   62.97%   63.03%   +0.06%     
==========================================
  Files         350      350              
  Lines       29990    29953      -37     
  Branches     3361     3352       -9     
==========================================
- Hits        18885    18881       -4     
+ Misses       9885     9856      -29     
+ Partials     1220     1216       -4

[chfast]

You can at least see in the coverage report that this code is not hit by any unit test. https://codecov.io/gh/ethereum/aleth/pull/5676/diff

[halfalicious]

Fixed CHANGELOG merge conflict