GetSubscription(): use per-object memory context.

GetSubscription(): use per-object memory context.

Constructing a Subcription object uses a number of small or temporary
allocations. Use a per-object memory context for easy cleanup.

Get rid of FreeSubscription() which did not free all the allocations
anyway. Also get rid of the PG_TRY()/PG_CATCH() logic in
ForeignServerConnectionString() which were used to avoid leaks during
GetSubscription().

Co-authored-by: Álvaro Herrera <alvherre@kurilemu.de>
Suggested-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/xvdjrdqnpap3uq7owbaox3r7p5gf7sv62aaqf2ju3vb6yglatr%40kvvwhoudrlxq
Discussion: https://postgr.es/m/CAA4eK1K=WjZ1maBCmj=5ZdO66AwPORK5ZBxVKedS0xdCcb621A@mail.gmail.com