pgsql: TransferPredicateLocksToNewTarget should initialize a new lock

TransferPredicateLocksToNewTarget should initialize a new lock
entry's commitSeqNo to that of the old one being transferred, or take
the minimum commitSeqNo if it is merging two lock entries.

Also, CreatePredicateLock should initialize commitSeqNo for to
InvalidSerCommitSeqNo instead of to 0. (I don't think using 0 would
actually affect anything, but we should be consistent.)

I also added a couple of assertions I used to track this down: a
lock's commitSeqNo should never be zero, and it should be
InvalidSerCommitSeqNo if and only if the lock is not held by
OldCommittedSxact.

Dan Ports, to fix leak of predicate locks reported by YAMAMOTO Takashi.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/dad1f4638235e5ff5696b948b88ba24cd99b415e

Modified Files
--------------
src/backend/storage/lmgr/predicate.c | 18 ++++++++++++++++--
1 files changed, 16 insertions(+), 2 deletions(-)