fix(p2p): fix update whitelist handler

[glevco]

Motivation

There are some Twisted errors that are logged raw, causing each log line to become an alert in OpsGenie, as in this incident: https://github.com/HathorNetwork/on-call-incidents/issues/153.

After some investigation, the reason for this is not 100% clear, but there are some suspicions. This PR attempts to fix at least one case that would cause the problem. We've tested a full node running on mainnet for a few days, and by inspecting its logs, it looks like the error has improved.

Acceptance Criteria

• Remove custom timeout handling in update_whitelist() to use Deferred.addTimeout().
• Move the readBody() call to be included in the timeout.
• Change update_whitelist() so the errback is after the callback, so the errback handles exceptions from the callback.
• Add unit tests for update_whitelist(), including cases for success, request error, and request timeout.

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: 3 lines in your changes are missing coverage. Please review.

Comparison is base (eaf76bd) 85.22% compared to head (baecd84) 85.34%.

Files	Patch %	Lines
hathor/p2p/manager.py	81.25%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #885      +/-   ##
==========================================
+ Coverage   85.22%   85.34%   +0.11%     
==========================================
  Files         285      285              
  Lines       22368    22362       -6     
  Branches     3371     3369       -2     
==========================================
+ Hits        19063    19084      +21     
+ Misses       2631     2609      -22     
+ Partials      674      669       -5     

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