crimson/os/seastore: implement OP_MERGE_COLLECTION

crimson/os/seastore: implement OP_MERGE_COLLECTION

Add a handler for Transaction::OP_MERGE_COLLECTION in SeaStore's
_do_transaction_step() and the corresponding _merge_collection()
implementation. Since coll_t is not part of the onode key, no
onode re-keying is needed: the operation updates the destination
collection's split_bits and removes the source collection from the
collection B-tree, all within a single transaction.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>