fuzz_group_decrypt.cpp got broken by 653790ea; fix it up

Return the base64-encoded length of pickles

Document the potential for message replays and possible mitigations

make olm_pickle_* return the lengths of the base64-encoded pickles, rather than
the raw pickle. (From the application's POV, the format of the pickle is
opaque: it doesn't even know that it is base64-encoded. So returning the length
of the raw pickle is particularly unhelpful.)

All the other methods clear their random inputs. This one needs to do the same,
to reduce the risk of the randomness being used elsewhere and leaking key info.

Return the message index when decrypting group messages.

Add notes on limitations to megolm spec

Applications can use the index to detect replays of the same message.

We're standardising on 'one-time keys' as a term for the thing that Bob uploads
for prekey messages.

Fix a buffer bounds check when decoding group messages

Add a README for the fuzzers

Fixes a segfault when a group message had exactly the length of the mac +
signature.

Also tweak skipping of unknown tags to avoid an extra trip around the loop.