Don't lock tables in RelationGetPartitionDispatchInfo.

Instead, lock them in the caller using find_all_inheritors so that
they get locked in the standard order, minimizing deadlock risks.

Also in RelationGetPartitionDispatchInfo, avoid opening tables which
are not partitioned; there's no need.

Amit Langote, reviewed by Ashutosh Bapat and Amit Khandekar

Discussion: http://postgr.es/m/91b36fa1-c197-b72f-ca6e-56c593bae68c@lab.ntt.co.jp