deparse: Take pg_catalog out of search_path
deparse: add support for CREATE RULE
bdr: avoid race conditions when installing bdr extensions

Previously installing the bdr (and prerequisite) extensions was racy
because only the sequencer installed them. Initially that wasn't much
of a problem since only the sequencer and manual queries needed the
schema, but that's not true anymore.

Instead check whether the schema exists in every started worker and
prevent races by acquiring a suitably strong lock on pg_extension
while checking/creating.
Initial cut at ALTER TABLE support

This supports ADD COLUMN.
Merge branch 'deparse' into bdr-next
bdr: mergme: Only add truncate trigger if CREATE TABLE
bdr: mergme: Also queue ALTER TABLE commands