crimson/.../lba_manager/btree: hold a reference to parent until added to cache

crimson/.../lba_manager/btree: hold a reference to parent until added to cache

Currently, we need to rely on the Transaction::read_set to ensure cache
residence of a leaf node until TransactionManager adds the logical
extent to the cache.  The next patch, however, will introduce a lazy
flag for Transaction's to enable doing snapshot inconsistent scans
without populating the read_set, so we'll want this to work without
it.

Signed-off-by: Samuel Just <sjust@redhat.com>