If a QE writer marks the transaction as aborted while a reader is
still executing the query the reader may affect shared memory state.
E.g. consider a transaction being aborted and a table needs to be
dropped as part of abort processing.  If the writer has dropped all
the buffers belonging to the table from shared buffer cache and is
about to unlink the file for the table.  Concurrently, a reader,
unaware of the writer's abort, is still executing the query.  The
reader may bring in a page from the file that the writer is about to
unlink into shared buffer cache.

In order to prevent such situations the writer walks procArray to find
the readers and sends SIGINT to them.

Walking procArray is expensive, and is avoided as much as possible.
The patch walks the procArray only if the command being aborted (or
the last command in a transaction that is being aborted) performed a
write and at least one reader slice was part of the query plan.

To avoid confusion on the QD due to "canceling MPP operation" error
messages emitted by the readers upon receiving the SIGINT, the readers
do not emit them on the libpq channel.

Discussion:
https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/S2WL1FtJEJ0/Wh6DfJ-RBwAJCo-authored-by: NEkta Khanna <ekhanna@pivotal.io>
Co-authored-by: NAsim R P <apraveen@pivotal.io>
Co-authored-by: NTaylor Vesely <tvesely@pivotal.io>