pgsql: Allocate separate DSM chunk for parallel Index[Only]Scan instrum

Allocate separate DSM chunk for parallel Index[Only]Scan instrumentation

Previously, parallel index and index-only scans packed the parallel scan
descriptor and shared instrumentation (for EXPLAIN ANALYZE) into a
single DSM allocation. Since scans may be instrumented without being
parallel-aware, and vice versa, using separate DSM chunks -- each with
its own TOC key -- is cleaner. A future commit will extend this pattern
to other scan node types.

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Tomas Vondra <tomas(at)vondra(dot)me>
Discussion: https://postgr.es/m/flat/a177a6dd-240b-455a-8f25-aca0b1c08c6e%40vondra.me

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/dd78e69cfc337f93cfc0303ddf660262d7f1237e

Modified Files
--------------
src/backend/access/index/indexam.c | 51 ++-----------
src/backend/executor/execParallel.c | 40 +++++++---
src/backend/executor/nodeBitmapIndexscan.c | 9 ++-
src/backend/executor/nodeIndexonlyscan.c | 114 +++++++++++++++++------------
src/backend/executor/nodeIndexscan.c | 114 +++++++++++++++++------------
src/include/access/genam.h | 7 +-
src/include/access/relscan.h | 1 -
src/include/executor/instrument_node.h | 8 ++
src/include/executor/nodeIndexonlyscan.h | 6 ++
src/include/executor/nodeIndexscan.h | 6 ++
10 files changed, 202 insertions(+), 154 deletions(-)