refactor(p2p): have two internal tiers for sync: available and enabled

[jansegre]

Motivation

This PR refactors internal code in order to make it easier to add sysctl controls for enabling and disabling specific sync versions.

Acceptance Criteria

• Refactor ConnectionsManager so that instead of having enable_sync_v1 and enable_sync_v2 it has "available sync versions" and "enabled sync versions". A sync version being available means that it can be enabled or disabled at runtime. This distinction is useful because there are init-time requirements for making specific sync versions available and it is practical to deal with these separately from whether a specific sync version is enabled or not.
• Make both sync-v1 and sync-v2 available by default, there's no CLI option to make them not available, CLI options only control whether they will be enabled, which can subsequently be controlled in runtime, but this will need a future PR that adds the sysctl controls.

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

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (0b32a31) 85.41% compared to head (830da1e) 85.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #867      +/-   ##
==========================================
- Coverage   85.41%   85.32%   -0.10%     
==========================================
  Files         281      281              
  Lines       22274    22306      +32     
  Branches     3375     3380       +5     
==========================================
+ Hits        19026    19032       +6     
- Misses       2581     2594      +13     
- Partials      667      680      +13     

Files	Coverage Δ
hathor/builder/builder.py	92.18% <100.00%> (+0.16%)	⬆️
hathor/builder/cli_builder.py	76.07% <100.00%> (+1.07%)	⬆️
hathor/p2p/states/hello.py	76.23% <100.00%> (ø)
hathor/p2p/manager.py	70.94% <59.37%> (-1.38%)	⬇️

... and 8 files with indirect coverage changes

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