Fix TransactionIdIsCurrentTransactionId() to use binary search instead of

Fix TransactionIdIsCurrentTransactionId() to use binary search instead of
linear search when checking child-transaction XIDs.  This makes for an
important speedup in transactions that have large numbers of children,
as in a recent example from Craig Ringer.  We can also get rid of an
ugly kluge that represented lists of TransactionIds as lists of OIDs.

Heikki Linnakangas