Merge #20197: p2p: protect onions in AttemptToEvictConnection(), add eviction protection test coverage

0cca08a8ee33b4e05ff586ae4fd914f5ea860cea Add unit test coverage for our onion peer eviction protection (Jon Atack)
caa21f586f951d626a67f391050c3644f1057f57 Protect onion+localhost peers in ProtectEvictionCandidatesByRatio() (Jon Atack)
8f1a53eb027727a4c0eaac6d82f0a8279549f638 Use EraseLastKElements() throughout SelectNodeToEvict() (Jon Atack)
8b1e156143740a5548dc7b601d40fb141e6aae1c Add m_inbound_onion to AttemptToEvictConnection() (Jon Atack)
72e30e8e03f880eba4bd1c3fc18b5558d8cef680 Add unit tests for ProtectEvictionCandidatesByRatio() (Jon Atack)
ca63b53ecdf377ce777fd959d400748912266748 Use std::unordered_set instead of std::vector in IsEvicted() (Jon Atack)
41f84d5eccd4c2620bf6fee616f2f8f717dbd6f6 Move peer eviction tests to a separate test file (Jon Atack)
f126cbd6de6e1a8fee0e900ecfbc14a88e362541 Extract ProtectEvictionCandidatesByRatio from SelectNodeToEvict (Jon Atack)

Pull request description:

  Now that #19991 and #20210 have been merged, we can determine inbound onion peers using `CNode::m_inbound_onion` and add it to the localhost peers protection in `AttemptToEvictConnection`, which was added in #19670 to address issue #19500.

  Update 28 February 2021: I've updated this to follow gmaxwell's suggestion in https://github.com/bitcoin/bitcoin/pull/20197#issuecomment-713865992.

  This branch now protects up to 1/4 onion peers (connected via our tor control service), if any, sorted by longest uptime. If any (or all) onion slots remain after that operation, they are then allocated to protect localhost peers, or a minimum of 2 localhost peers in the case that no onion slots remain and 2 or more onion peers were protected, sorted as before by longest uptime.

  This patch also adds test coverage for the longest uptime, localhost, and onion peer eviction protection logic to build on the welcome initial unit testing of #20477.

  Suggest reviewing the commits that move code with `colorMoved = dimmed-zebra` and `colorMovedWs = allow-indentation-change`.

  Closes #11537.

ACKs for top commit:
  laanwj:
    Code review ACK 0cca08a8ee33b4e05ff586ae4fd914f5ea860cea
  vasild:
    ACK 0cca08a8ee33b4e05ff586ae4fd914f5ea860cea

Tree-SHA512: 2f5a63f942acaae7882920fc61f0185dcd51da85e5b736df9d1fc72343726dd17da740e02f30fa5dc5eb3b2d8345707aed96031bec143d48a2497a610aa19abd

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