pgsql: Allow EXPLAIN (ANALYZE, VERBOSE) to display per-worker statistic

Allow EXPLAIN (ANALYZE, VERBOSE) to display per-worker statistics.

The original parallel sequential scan commit included only very limited
changes to the EXPLAIN output. Aggregated totals from all workers were
displayed, but there was no way to see what each individual worker did
or to distinguish the effort made by the workers from the effort made by
the leader.

Per a gripe by Thom Brown (and maybe others). Patch by me, reviewed
by Amit Kapila.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b287df70e4080350aa471ecca428be145581dd4d

Modified Files
--------------
src/backend/commands/explain.c | 251 ++++++++++++++++++++++-------------
src/backend/executor/execParallel.c | 98 ++++++++------
src/include/executor/instrument.h | 6 +
src/include/nodes/execnodes.h | 1 +
4 files changed, 226 insertions(+), 130 deletions(-)