refactor(p2p): rename sync *manager* to sync *agent*

[jansegre]

Motivation

We've been using the term "manager" for the class that deals with the sync from the point of view of an individual connection but we've already agreed to changing it to "agent" instead. This PR finishes the transition to this term.

Acceptance Criteria

• Use sync_agent/SyncAgent* instead of sync_manager/SyncManager* everywhere throughout the code.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Merging #729 (1d48776) into master (e9235e6) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #729      +/-   ##
==========================================
- Coverage   84.43%   84.34%   -0.09%     
==========================================
  Files         251      251              
  Lines       21586    21586              
  Branches     2917     2917              
==========================================
- Hits        18226    18207      -19     
- Misses       2730     2745      +15     
- Partials      630      634       +4     

Files Changed	Coverage Δ
hathor/p2p/manager.py	72.65% <100.00%> (+0.75%)	⬆️
hathor/p2p/protocol.py	92.98% <100.00%> (ø)
hathor/p2p/resources/status.py	100.00% <100.00%> (ø)
hathor/p2p/states/ready.py	93.47% <100.00%> (ø)
hathor/p2p/sync_agent.py	100.00% <100.00%> (ø)
hathor/p2p/sync_factory.py	87.50% <100.00%> (ø)
hathor/p2p/sync_v1/agent.py	85.45% <100.00%> (+0.44%)	⬆️
hathor/p2p/sync_v1/factory_v1_0.py	72.22% <100.00%> (ø)
hathor/p2p/sync_v1/factory_v1_1.py	100.00% <100.00%> (ø)
hathor/p2p/sync_v2/factory.py	100.00% <100.00%> (ø)
... and 3 more