Merge #17428: p2p: Try to preserve outbound block-relay-only connections during restart

a490d074b3 doc: Add anchors.dat to files.md (Hennadii Stepanov)
0a85e5a7bc p2p: Try to connect to anchors once (Hennadii Stepanov)
5543c7ab28 p2p: Fix off-by-one error in fetching address loop (Hennadii Stepanov)
4170b46544 p2p: Integrate DumpAnchors() and ReadAnchors() into CConnman (Hennadii Stepanov)
bad16aff49 p2p: Add CConnman::GetCurrentBlockRelayOnlyConns() (Hennadii Stepanov)
c29272a157 p2p: Add ReadAnchors() (Hennadii Stepanov)
567008d2a0 p2p: Add DumpAnchors() (Hennadii Stepanov)

Pull request description:

  This is an implementation of #17326:
  - all (currently 2) outbound block-relay-only connections (#15759) are dumped to `anchors.dat` file
  - on restart a node tries to connect to the addresses from `anchors.dat`

  This PR prevents a type of eclipse attack when an attacker exploits a victim node restart to force it to connect to new, probably adversarial, peers.

ACKs for top commit:
  jnewbery:
    code review ACK a490d074b3
  laanwj:
    Code review ACK a490d074b3

Tree-SHA512: 0f5098a3882f2814be1aa21de308cd09e6654f4e7054b79f3cfeaf26bc02b814ca271497ed00018d199ee596a8cb9b126acee8b666a29e225b08eb2a49b02ddd

doc/files.md

@@ -50,6 +50,7 @@ Subdirectory       | File(s)               | Description
 `indexes/blockfilter/basic/db/` | LevelDB database      | Blockfilter index LevelDB database for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `indexes/blockfilter/basic/`    | `fltrNNNNN.dat`<sup>[\[2\]](#note2)</sup> | Blockfilter index filters for the basic filtertype; *optional*, used if `-blockfilterindex=basic`
 `wallets/`         |                       | [Contains wallets](#multi-wallet-environment); can be specified by `-walletdir` option; if `wallets/` subdirectory does not exist, wallets reside in the [data directory](#data-directory-location)
+`./`               | `anchors.dat`         | Anchor IP address database, created on shutdown and deleted at startup. Anchors are last known outgoing block-relay-only peers that are tried to re-connect to on startup
 `./`               | `banlist.dat`         | Stores the IPs/subnets of banned nodes
 `./`               | `bitcoin.conf`        | User-defined [configuration settings](bitcoin-conf.md) for `bitcoind` or `bitcoin-qt`. File is not written to by the software and must be created manually. Path can be specified by `-conf` option
 `./`               | `bitcoind.pid`        | Stores the process ID (PID) of `bitcoind` or `bitcoin-qt` while running; created at start and deleted on shutdown; can be specified by `-pid` option