pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/56163004b8b2151db279744b77138d4d90e2d5cb

Modified Files
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src/backend/catalog/heap.c       | 16 ++++++++++++++--
src/backend/catalog/partition.c  |  7 -------
src/backend/commands/tablecmds.c | 25 +++++++------------------
3 files changed, 21 insertions(+), 27 deletions(-)