In this commit, we ensure that the test for migration 9 uses the same
encoding/decoding functions as was present in the repo when the
migration was first added. Otherwise, the test will fail as it'll try to
use the decoding functions of master (migration 10 and onwards) rather
than the decoding function of migration 9.
In this commit, we introduce a migration for the message store
sub-bucket that will migrate all keys within it to a new key format.
This new key format is composed of the peer's public key, followed by
the short channel ID, followed by the message type. This migration is
needed in order to provide backwards-compatibility with messages that
were previously stored before the introduction of the new key format.
