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

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@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/flat/a177a6dd-240b-455a-8f25-aca0b1c08c6e%40vondra.me