Remove HeapTuple and Buffer dependency for predicate locking functions

Proposing following changes to make predicate locking and checking
functions generic and remove dependency on HeapTuple and Heap AM. We
made these changes to help with Zedstore. I think the changes should
help Zheap and other AMs in general.

- Change PredicateLockTuple() to PredicateLockTID(). So, instead of
passing HeapTuple to it, just pass ItemPointer and tuple insert
transaction id if known. This was also discussed earlier in [1],
don't think was done in that context but would be helpful in future
if such requirement comes up as well.

- CheckForSerializableConflictIn() take blocknum instead of
buffer. Currently, the function anyways does nothing with the buffer
just needs blocknum. Also, helps to decouple dependency on buffer as
not all AMs may have one to one notion between blocknum and single
buffer. Like for zedstore, tuple is stored across individual column
buffers. So, wish to have way to lock not physical buffer but
logical blocknum.

- CheckForSerializableConflictOut() no more takes HeapTuple nor
buffer, instead just takes xid. Push heap specific parts from
CheckForSerializableConflictOut() into its own function
HeapCheckForSerializableConflictOut() which calls
CheckForSerializableConflictOut(). The alternative option could be
CheckForSerializableConflictOut() take callback function and
callback arguments, which gets called if required after performing
prechecks. Though currently I fell AM having its own wrapper to
perform AM specific task and then calling
CheckForSerializableConflictOut() is fine.

Attaching patch which makes these changes.

This way PredicateLockTID(), CheckForSerializableConflictIn() and
CheckForSerializableConflictOut() functions become usable by any AM.

1]
https://www.postgresql.org/message-id/CAEepm%3D2QbqQ_%2BKQQCnhKukF6NEAeq4SqiO3Qxe%2BfHza5-H-jKA%40mail.gmail.com