refactor(sync-v2): rename SyncAgent module from manager to agent

[jansegre]

Motivation

After the last refactor this module name was left unchanged.

Acceptance Criteria

• Rename hathor.p2p.sync_v2.manager module to hathor.p2p.sync_v2.agent adjusting all the references.

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 #748 (413a42c) into master (a63ab5f) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

❗ Current head 413a42c differs from pull request most recent head 4277cc2. Consider uploading reports for the commit 4277cc2 to get more accurate results

@@            Coverage Diff             @@
##           master     #748      +/-   ##
==========================================
- Coverage   84.57%   84.48%   -0.09%     
==========================================
  Files         253      253              
  Lines       21708    21706       -2     
  Branches     2941     2941              
==========================================
- Hits        18359    18338      -21     
- Misses       2710     2724      +14     
- Partials      639      644       +5     

Files Changed	Coverage Δ
hathor/p2p/sync_v2/agent.py	74.38% <ø> (ø)
hathor/p2p/sync_v2/mempool.py	94.11% <ø> (ø)
hathor/p2p/sync_v2/streamers.py	88.41% <ø> (ø)
hathor/p2p/sync_v2/factory.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes