[bugfix] commit timestamps ERROR on lookup of FrozenTransactionId

Hi all

Today I ran into an issue where commit timestamp lookups were failing with

        ERROR: cannot retrieve commit timestamp for transaction 2

which is of course FrozenTransactionId.

TransactionIdGetCommitTsData(...) ERRORs on !TransactionIdIsNormal(),
which I think is wrong. Attached is a patch to make it return 0 for
FrozenTransactionId and BootstrapTransactionId, like it does for xids
that are too old.

Note that the prior behaviour was as designed and has tests to enforce
it. I just think it's wrong, and it's also not documented.

IMO this should be back-patched to 9.6 and, without the TAP test part, to 9.5.

-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services