Merge #20284: addrman: ensure old versions don't parse peers.dat

38ada892ed addrman: ensure old versions don't parse peers.dat (Vasil Dimov)

Pull request description:

  Even though the format of `peers.dat` was changed in a backwards
  incompatible way, it is not guaranteed that old versions will fail to
  parse it. There is a chance that old versions parse its contents as
  garbage and use it.

  Old versions expect the "key size" field to be 32 and fail the parsing
  if it is not. Thus, we put something other than 32 in it. This will make
  versions between 0.11.0 and 0.20.1 deterministically fail on the new
  format. Versions prior to https://github.com/bitcoin/bitcoin/pull/5941
  will still parse it as garbage.

  Also, introduce a way to increment the `peers.dat` format in a way that
  does not necessary make older versions refuse to read it.

ACKs for top commit:
  jnewbery:
    ACK 38ada892ed
  laanwj:
    Code review ACK 38ada892ed
  MarcoFalke:
    re-ACK 38ada892ed 🥐

Tree-SHA512: 550bd660c5019dba0f9c334aca8a11c4a0463cfddf11efe7a4a5585ffb05549c82b95066fba5d073ae37893e0eccc158a7ffea9b33ea031d9be4a39e44f6face

doc/release-notes.md

@@ -65,8 +65,8 @@ format of this file has been changed in a backwards-incompatible way in order to
 accommodate the storage of Tor v3 and other BIP155 addresses. This means that if
 the file is modified by 0.21.0 or newer then older versions will not be able to
 read it. Those old versions, in the event of a downgrade, will log an error
-message that deserialization has failed and will continue normal operation
-as if the file was missing, creating a new empty one. (#19954)
+message "Incorrect keysize in addrman deserialization" and will continue normal
+operation as if the file was missing, creating a new empty one. (#19954)
 
 Notable changes
 ===============