1. We add a vector of locks to trx_t. This array contains the autoinc
  locks granted to a transaction. There is one per table.

  2. We enforce releasing of these locks in the reverse order from the
  one in which they are acquired. The assumption is that since the
  AUTOINC locks are statement level locks. Nested statements introduced
  by triggers are stacked it should hold.

There was some cleanup done to the vector code too by adding const and
some new functions. Rename dict_table_t::auto_inc_lock to autoinc_lock.

Fix Bug#26316 Triggers create duplicate entries on auto-increment columns
rb://22