pgsql: Allow a parallel context to relaunch workers.

Allow a parallel context to relaunch workers.

This may allow some callers to avoid the overhead involved in tearing
down a parallel context and then setting up a new one, which means
releasing the DSM and then allocating and populating a new one. I
suspect we'll want to revise the Gather node to make use of this new
capability, but even if not it may be useful elsewhere and requires
very little additional code.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b0b0d84b3d663a148022e900ebfc164284a95f55

Modified Files
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src/backend/access/transam/README.parallel | 5 +++
src/backend/access/transam/parallel.c | 49 ++++++++++++++++++++++++++++
src/include/access/parallel.h | 1 +
3 files changed, 55 insertions(+)