Implement same-table concurrency using Postgres CTID column. (#410)

* Implement same-table concurrency using Postgres CTID column.

The Postgres system column "ctid" is the physical location of the row
version within its table. It is encoded using the tid datatype:
`(pagenumber,rownumber)`.

Every single Postgres table has a "ctid" column and it is always possible to
split a table's contents by using ctid based ranges. The question of this
approach being good at reducing pgcopydb timings remains open: this PR is
meant for allowing more experimentations.

* Restrict CTID COPY partitionning to tables using "heap" am.

TID scan might not be supported by other Table AM (such as Citus Columnar).