Reduce ibuf_mutex and ibuf_pessimistic_insert_mutex contention further.

Protect ibuf->empty by the insert buffer root page latch, not ibuf_mutex.

ibuf_tree_root_get(): Assert that ibuf_mutex is owned by the
caller. Assert that the stamped page number is correct. Assert that
ibuf->empty agrees with the root page.

ibuf_size_update(): Do not update ibuf->empty.

ibuf_init_at_db_start(): Update ibuf->empty while holding the root page latch.

ibuf_add_free_page(): Return TRUE/FALSE instead of DB_SUCCESS/DB_STRONG_FAIL.

ibuf_remove_free_page(): Release ibuf_pessimistic_insert_mutex as
early as possible.

ibuf_contract_ext(): Rely on a dirty read of ibuf->empty, unless the
server is being shut down. Never acquire ibuf_mutex. Eliminate n_stored.

ibuf_contract_after_insert(): Never acquire ibuf_mutex. Perform dirty
reads of ibuf->size and ibuf->max_size.

ibuf_insert_low(): Only acquire ibuf_mutex for mode==BTR_MODIFY_TREE.
Perform dirty reads of ibuf->size and ibuf->max_size. Update
ibuf->empty while holding the root page latch.

ibuf_delete_rec(): Update ibuf->empty while holding the root page latch.

ibuf_is_empty(): Release ibuf_mutex earlier.