trx_t::autoinc_locks causes unnecessary dynamic memory allocation

The data structure trx_t::autoinc_locks is of type ib_vector_t*. It is unconditionally allocated in trx_create() even when no AUTO_INCREMENT locks will be used by the transaction, even for read-only transactions.

We had better make use of a template that was introduced in MDEV-30289:

small_vector<lock_t*, 4> autoinc_locks;

The data elements are trivially assignable and allocated separately; the first 8 table locks of a transaction would use the trx->lock.table_pool in lock_table_create(). Any data-modifying transaction will typically also require a table IX lock in addition to the auto-increment lock. That is why 4 would be a reasonable fixed size.

Only if a transaction involved more than 4 auto-increment locks, we would dynamically allocate a larger vector, similar to how the 16-element mtr_t::m_memo works.