Merge #15759: p2p: Add 2 outbound block-relay-only connections

0ba08020c9791f7caf5986ad6490c16a2b66cd83 Disconnect peers violating blocks-only mode (Suhas Daftuar)
937eba91e1550bc3038dc541c236ac83e0a0e6d5 doc: improve comments relating to block-relay-only peers (Suhas Daftuar)
430f489027f15c1e4948ea4378954df24e3fee88 Don't relay addr messages to block-relay-only peers (Suhas Daftuar)
3a5e885306ea954d7eccdc11502e91a51dab8ec6 Add 2 outbound block-relay-only connections (Suhas Daftuar)
b83f51a4bbe29bf130a2b0c0e85e5bffea107f75 Add comment explaining intended use of m_tx_relay (Suhas Daftuar)
e75c39cd425f8c4e5b6bbb2beecb9c80034fefe1 Check that tx_relay is initialized before access (Suhas Daftuar)
c4aa2ba82211ea5988ed7fe21e1b08bc3367e6d4 [refactor] Change tx_relay structure to be unique_ptr (Suhas Daftuar)
4de0dbac9b286c42a9b10132b7c2d76712f1a319 [refactor] Move tx relay state to separate structure (Suhas Daftuar)
26a93bce29fd813e1402b013f402869c25b656d1 Remove unused variable (Suhas Daftuar)

Pull request description:

  Transaction relay is optimized for a combination of redundancy/robustness as well as bandwidth minimization -- as a result transaction relay leaks information that adversaries can use to infer the network topology.

  Network topology is better kept private for (at least) two reasons:

  (a) Knowledge of the network graph can make it easier to find the source IP of a given transaction.

  (b) Knowledge of the network graph could be used to split a target node or nodes from the honest network (eg by knowing which peers to attack in order to achieve a network split).

  We can eliminate the risks of (b) by separating block relay from transaction relay; inferring network connectivity from the relay of blocks/block headers is much more expensive for an adversary.

  After this commit, bitcoind will make 2 additional outbound connections that are only used for block relay. (In the future, we might consider rotating our transaction-relay peers to help limit the effects of (a).)

ACKs for top commit:
  sipa:
    ACK 0ba08020c9791f7caf5986ad6490c16a2b66cd83
  ajtowns:
    ACK 0ba08020c9791f7caf5986ad6490c16a2b66cd83 -- code review, ran tests. ran it on mainnet for a couple of days with MAX_BLOCKS_ONLY_CONNECTIONS upped from 2 to 16 and didn't observe any unexpected behaviour: it disconnected a couple of peers that tried sending inv's, and it successfully did compact block relay with some block relay peers.
  TheBlueMatt:
    re-utACK 0ba08020c9791f7caf5986ad6490c16a2b66cd83. Pointed out that stats.fRelayTxes was sometimes uninitialized for blocksonly peers (though its not a big deal and only effects RPC), which has since been fixed here. Otherwise changes are pretty trivial so looks good.
  jnewbery:
    utACK 0ba08020c9791f7caf5986ad6490c16a2b66cd83
  jamesob:
    ACK https://github.com/bitcoin/bitcoin/commit/0ba08020c9791f7caf5986ad6490c16a2b66cd83

Tree-SHA512: 4c3629434472c7dd4125253417b1be41967a508c3cfec8af5a34cad685464fbebbb6558f0f8f5c0d4463e3ffa4fa3aabd58247692cb9ab8395f4993078b9bcdf

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