Sync traffic optimization

[arkpar]

• Prevent downloading subchain headers from all available peers.
• Abort pending download requests properly.

[gavofyork]

would be nice to comprehend this PR a bit. maybe elaborate on the additional state (what does it mean? when should they be in particular configurations? how does the new state relate to existing state? are there any invalid combinations?) and logic (why is the state being changed in the way it is? is there anything special about the condition that makes this state change valid now? what kind of post-conditions are being achieved?).

[arkpar]

There is no new state or state change introduced here.
There are 3 major changes here:

1. Additional validation for incoming headers. When we get a collection of subchain headers we check that at least one of them is known before inserting to H
2. Request subchain header from at most 5 peers.
3. When block downloader moves from downloading subchain heads to blocks all outstanding downloads are marked as expired.

So the state changes are:

1. There can be no more than 5 peers downloading subchain heads. The rationale is to save traffic.

[gavofyork]

gitlab CI fails build.