Merges changesets from the source replica into the target replica.
Fails fatally if the source and target are the same, or the target is not writable.
Conflicts are resolved by either the resolver specified in the
PROPHET_RESOLVER environmental variable,
prefer argument (can be set to
in which case Prophet will always prefer changesets from one replica or the other),
or by using a default resolver.
Returns the number of changesets merged.