Problem:

The autoinc_mutex member of the dict_table_t is not required for all the
tables.  But this is being created for all the dict_table_t objects.
Similar problem exists for dict_index_t::zip_pad::mutex.

Solution:

Make use of the lazy creation approach, whereby the autoinc_mutex member
will be allocated and initialized when there is a first lock attempt.
Apply same solution to dict_index_t::zip_pad::mutex.

rb#7415 approved by Vasil.