pgsql: 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(at)kurilemu(dot)de>
Suggested-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Discussion: https://postgr.es/m/xvdjrdqnpap3uq7owbaox3r7p5gf7sv62aaqf2ju3vb6yglatr%40kvvwhoudrlxq
Discussion: https://postgr.es/m/CAA4eK1K=WjZ1maBCmj=5ZdO66AwPORK5ZBxVKedS0xdCcb621A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f16f5d608ca6830f452a3b1e9f72afc6851cab38

Modified Files
--------------
src/backend/catalog/pg_subscription.c | 24 +++++-------
src/backend/foreign/foreign.c | 66 +++++++++-----------------------
src/backend/replication/logical/worker.c | 36 ++++++++---------
src/include/catalog/pg_subscription.h | 3 +-
4 files changed, 48 insertions(+), 81 deletions(-)