CopyFrom() needs a range table for formatting certain errors for
constraint violations.

This changes the mechanism of how the range table is passed to the
CopyFrom() executor state.  We used to generate the range table and one
entry for the relation manually inside DoCopy().  Now we use
addRangeTableEntryForRelation() to setup the range table and relation
entry for the ParseState, which is then passed down by BeginCopyFrom().

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Reported-by: Euler Taveira <euler@timbira.com.br>