In semi-consistent read, only unlock freshly locked non-matching records.

lock_rec_lock_fast(): Return LOCK_REC_SUCCESS,
LOCK_REC_SUCCESS_CREATED, or LOCK_REC_FAIL instead of TRUE/FALSE.

enum db_err: Add DB_SUCCESS_LOCKED_REC for indicating a successful
operation where a record lock was created.

lock_sec_rec_read_check_and_lock(),
lock_clust_rec_read_check_and_lock(), lock_rec_enqueue_waiting(),
lock_rec_lock_slow(), lock_rec_lock(), row_ins_set_shared_rec_lock(),
row_ins_set_exclusive_rec_lock(), sel_set_rec_lock(),
row_sel_get_clust_rec_for_mysql(): Return DB_SUCCESS_LOCKED_REC if a
new record lock was created. Adjust callers.

row_unlock_for_mysql(): Correct the function documentation.

row_prebuilt_t::new_rec_locks: Correct the documentation.