Merge bitcoin/bitcoin#21261: p2p: update inbound eviction protection for multiple networks, add I2P peers

1b1088d52fbff8b1c9438d6aa8c6edcbdd471457 test: add combined I2P/onion/localhost eviction protection tests (Jon Atack)
7c2284eda22a08dbf2a560894e496e245d026ee0 test: add tests for inbound eviction protection of I2P peers (Jon Atack)
ce02dd1ef1f7f54f33780b32f195d31c1cc87318 p2p: extend inbound eviction protection by network to I2P peers (Jon Atack)
70bbc62711643ec57cce620f9f7a0e1fe5fb6346 test: add combined onion/localhost eviction protection coverage (Jon Atack)
045cb40192bf3dfa6c42916237e55f86bbc788d4 p2p: remove unused m_is_onion member from NodeEvictionCandidate struct (Jon Atack)
310fab49282d507e5fa710afb20d036604bbf3a2 p2p: remove unused CompareLocalHostTimeConnected() (Jon Atack)
9e889e8a5c021b0ec7e4c4d17d418ab4a0accad4 p2p: remove unused CompareOnionTimeConnected() (Jon Atack)
787d46bb2a39fb39166882cc6e0afbc34424d88e p2p: update ProtectEvictionCandidatesByRatio() doxygen docs (Jon Atack)
1e15acf478ae071234350c9b38dc823dfe2e3419 p2p: make ProtectEvictionCandidatesByRatio() fully ratio-based (Jon Atack)
3f8105c4d251e0e81bdd31f0999004e36f8990b2 test: remove combined onion/localhost eviction protection tests (Jon Atack)
38a81a8e20b0e5ad9fef0eae8abd914619f05b25 p2p: add CompareNodeNetworkTime() comparator struct (Jon Atack)
4ee7aec47ebf6b59b4d930e6e4025e91352c05b4 p2p: add m_network to NodeEvictionCandidate struct (Jon Atack)
7321e6f2fe1641eb331f30e68646f5984d4bcbb3 p2p, refactor: rename vEvictionCandidates to eviction_candidates (Jon Atack)
ec590f1d91325404383d74098a5b42a2cd67dad9 p2p, refactor: improve constness in ProtectEvictionCandidatesByRatio() (Jon Atack)
4a19f501abac4adb476a6f2a30dfdf1a35892ccc test: add ALL_NETWORKS to test utilities (Jon Atack)
519e76bb64d03ecac175ec33c31e37d0e90f037f test: speed up and simplify peer_eviction_test (Jon Atack)
1cde8005233d163723d4d5bf1bacf22e6cb7a07e p2p, refactor: rm redundant erase_size calculation in SelectNodeToEvict() (Jon Atack)

Pull request description:

  Continuing the work in #20197 and #20685, this pull updates and abstracts our inbound eviction protection to make it fully ratio-based and easily extensible to peers connected via high-latency privacy networks that we newly support, like I2P and perhaps others soon, as these peers are disadvantaged by the latency criteria of our eviction logic.

  It then adds eviction protection for peers connected over I2P.  As described in https://github.com/bitcoin/bitcoin/pull/20685#issuecomment-767486499, we've observed over the past few months that I2P peers have a min ping latency similar to or greater than that of onion peers.

  The algorithm is a basically a multi-pass knapsack:

  - Count the number of eviction candidates in each of the disadvantaged
    privacy networks.

  - Sort the networks from lower to higher candidate counts, so that
    a network with fewer candidates will have the first opportunity
    for any unused slots remaining from the previous iteration.  In
    the case of a tie in candidate counts, priority is given by array
    member order from first to last, guesstimated to favor more unusual
    networks.

  - Iterate through the networks in this order.  On each iteration,
    allocate each network an equal number of protected slots targeting
    a total number of candidates to protect, provided any slots remain
    in the knapsack.

  - Protect the candidates in that network having the longest uptime,
    if any in that network are present.

  - Continue iterating as long as we have non-allocated slots
    remaining and candidates available to protect.

  The goal of this logic is to favorise the diversity of our peer connections.

  The individual commit messages describe each change in more detail.

  Special thank you to Vasil Dimov for the excellent review feedback and the algorithm improvement that made this change much better than it would have been otherwise. Thanks also to Antoine Riard, whose review feedback nudged this change to protect disadvantaged networks having fewer, rather than more, eviction candidates.

ACKs for top commit:
  laanwj:
    Code review re-ACK 1b1088d52fbff8b1c9438d6aa8c6edcbdd471457
  vasild:
    ACK 1b1088d52fbff8b1c9438d6aa8c6edcbdd471457

Tree-SHA512: 722f790ff11f2969c79e45a5e0e938d94df78df8687e77002f32e3ef5c72a9ac10ebf8c7a9eb7f71882c97ab0e67b2778191effdb747d9ca54d7c23c2ed19a90

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