Improve performance of EXPLAIN with large range tables.

 Improve performance of EXPLAIN with large range tables.
 
 As of 9.3, ruleutils.c goes to some lengths to ensure that table and column
 aliases used in its output are unique.  Of course this takes more time than
 was required before, which in itself isn't fatal.  However, EXPLAIN was set
 up so that recalculation of the unique aliases was repeated for each
 subexpression printed in a plan.  That results in O(N^2) time and memory
 consumption for large plan trees, which did not happen in older branches.
 
 Fortunately, the expensive work is the same across a whole plan tree,
 so there is no need to repeat it; we can do most of the initialization
 just once per query and re-use it for each subexpression.  This buys
 back most (not all) of the performance loss since 9.2.
 
 We need an extra ExplainState field to hold the precalculated deparse
 context.  That's no problem in HEAD, but in the back branches, expanding
 sizeof(ExplainState) seems risky because third-party extensions might
 have local variables of that struct type.  So, in 9.4 and 9.3, introduce
 an auxiliary struct to keep sizeof(ExplainState) the same.  We should
 refactor the APIs to avoid such local variables in future, but that's
 material for a separate HEAD-only commit.
 
 Per gripe from Alexey Bashtanov.  Back-patch to 9.3 where the issue
 was introduced.