pgsql: Fix bug in early startup of Hot Standby with subtransactions.

Fix bug in early startup of Hot Standby with subtransactions.
When HS startup is deferred because of overflowed subtransactions, ensure
that we re-initialize KnownAssignedXids for when both existing and incoming
snapshots have non-zero qualifying xids.

Fixes bug #6661 reported by Valentine Gogichashvili.

Analysis and fix by Andres Freund

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/37255705399b57b2e413814799f0bc9b94fda14a

Modified Files
--------------
src/backend/storage/ipc/procarray.c |   32 +++++++++++++++++++++++++++++++-
1 files changed, 31 insertions(+), 1 deletions(-)