Clean up recover().

recover() panics if things go wrong: don't
return any  value from it (until we fix it to not panic).

In recover() current_wal is always NULL, since we run
it right after start (or after establishing a connection
with a replica). Don't check for current_wal.

Don't produce a stupid error message "can't find WAL
with LSN:" when recovering from a snapshot, and a snapshot
is all we have in the data directory.

We (ab)use recover() and recover_remaining_wals() in
replication and in initial recovery, without doing the
abstraction homework, and this leads to convoluted
code and redundant logic.