XA specification doesn't permit empty gtrid. It is now enforced by this
patch. This solution was agreed in favour of fixing InnoDB, which doesn't
expect empty XID since early 10.5.
Also fixed wrong assertion (and added a test cases) that didn't permit
64 bytes gtrid + 64 bytes bqual.
In InnoDB, an INSERT will not create an explicit lock object. Instead,
the inserted record is initially implicitly locked by the transaction
that wrote its trx_t::id to the hidden system column DB_TRX_ID.
(Other transactions would check if DB_TRX_ID is referring to a
transaction that has not been committed.)
If a record was inserted in the current transaction, it would be
implicitly locked by that transaction. Only if some other transaction
is requesting access to the record, the implicit lock should be
converted to an explicit one, so that the waits-for graph can be
constructed for detecting deadlocks and lock wait timeouts.
Before this fix, InnoDB would convert implicit locks to
explicit ones, even if no conflict exists.
lock_rec_convert_impl_to_expl(): Return whether caller_trx
already holds an explicit lock that covers the record.
row_vers_impl_x_locked_low(): Avoid a lookup if the record matches
caller_trx->id.
lock_trx_has_expl_x_lock(): Renamed from lock_trx_has_rec_x_lock().
row_upd_clust_step(): In a debug assertion, check for implicit lock
before invoking lock_trx_has_expl_x_lock().
rw_trx_hash_t::find(): Make do_ref_count a mandatory parameter.
Assert that trx_id is not 0 (the caller should check it).
trx_sys_t::is_registered(): Only invoke find() if id != 0.
trx_sys_t::find(): Add the optional parameter do_ref_count.
lock_rec_queue_validate(): Avoid lookup for trx_id == 0.
