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

Even though the format of `peers.dat` was changed in an incompatible
way (old software versions <0.21 cannot understand the new file format),
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
(<0.11.0) 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.

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