Implement List support for TransactionId
Use it for RelationSyncEntry->streamed_txns, which is currently using an
integer list.
The API support is not complete, not because it is hard to write but
because it's unclear that it's worth the code space, there being so
little use of XID lists.
Discussion: https://postgr.es/m/
202205130830.g5ntonhztspb@alvherre.pgsql
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>