feat(sync-v2): sync-v2 implemented, sync-v1 still default

[jansegre]

These changes were separated from #236, it should now have mostly the sync-v2 base code itself.

[jansegre]

Parts of the changes on this PR, and many of them due to feedback from the review, have been moved to #292

[codecov[bot]]

Codecov Report

Merging #275 (a9dc05b) into master (175f40a) will increase coverage by 0.26%. The diff coverage is 77.58%.

:exclamation: Current head a9dc05b differs from pull request most recent head a7581ef. Consider uploading reports for the commit a7581ef to get more accurate results

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   84.02%   84.28%   +0.26%     
==========================================
  Files         247      251       +4     
  Lines       20625    21602     +977     
  Branches     2784     2917     +133     
==========================================
+ Hits        17330    18208     +878     
- Misses       2717     2763      +46     
- Partials      578      631      +53     

Files Changed	Coverage Δ
hathor/indexes/rocksdb_deps_index.py	58.77% <33.33%> (+0.23%)	:arrow_up:
hathor/p2p/sync_v2/manager.py	72.61% <72.61%> (ø)
hathor/manager.py	82.68% <81.25%> (+5.22%)	:arrow_up:
hathor/p2p/sync_v2/streamers.py	88.41% <88.41%> (ø)
hathor/indexes/memory_deps_index.py	87.12% <92.00%> (+15.53%)	:arrow_up:
hathor/p2p/sync_v2/mempool.py	94.11% <94.11%> (ø)
hathor/indexes/deps_index.py	90.00% <100.00%> (-0.57%)	:arrow_down:
hathor/p2p/manager.py	72.65% <100.00%> (+0.06%)	:arrow_up:
hathor/p2p/messages.py	100.00% <100.00%> (ø)
hathor/p2p/sync_v2/factory.py	100.00% <100.00%> (ø)
... and 2 more

... and 14 files with indirect coverage changes