Fix crash on multi writers with wal_flush_before_commit option

- If wal_flush_before_commit option is turned on, WAL is flushed before
fdb_commit() is invoked. The updated HB+trie nodes are still writable
unless fdb_commit(), so that race condition can occur if concurrent
writers modify the node at the same time.

- The (writable) dirty index nodes are shared among concurrent writers.