Follow up on wal-fsync0-merge branch.

Fix bug
https://bugs.launchpad.net/tarantool/+bug/983094
"Valgrind tests were broken by WAL writer patch"

Keep track of WAL files opened for write
in recovery

Extend atfork handler to clear the wals,
to avoid collision with reuse of recovery
to save a snapshot.

Move some WAL-specific configuration settings
from wal_class to recovery, to avoid layers
of indirection when doing RELOAD CONFIGURATION