Fix relcache handling of the 'default' partition

 Fix relcache handling of the 'default' partition
 
 My commit 4dba331cb3dc that moved around CommandCounterIncrement calls
 in partitioning DDL code unearthed a problem with the relcache handling
 for the 'default' partition: the construction of a correct relcache
 entry for the partitioned table was at the mercy of lack of CCI calls in
 non-trivial amounts of code.  This was prone to creating problems later
 on, as the code develops.  This was visible as a test failure in a
 compile with RELCACHE_FORCE_RELASE (buildfarm member prion).
 
 The problem is that after the mentioned commit it was possible to create
 a relcache entry that had incomplete information regarding the default
 partition because I introduced a CCI between adding the catalog entries
 for the default partition (StorePartitionBound) and the update of
 pg_partitioned_table entry for its parent partitioned table
 (update_default_partition_oid).  It seems the best fix is to move the
 latter so that it occurs inside the former; the purposeful lack of
 intervening CCI should be more obvious, and harder to break.
 
 I also remove a check in RelationBuildPartitionDesc that returns NULL if
 the key is not set.  I couldn't find any place that needs this hack
 anymore; probably it was required because of bugs that have since been
 fixed.
 
 Fix a few typos I noticed while reviewing the code involved.
 
 Discussion: https://postgr.es/m/20180320182659.nyzn3vqtjbbtfgwq@alvherre.pgsql