pgsql: Add new function WaitForParallelWorkersToAttach.

Add new function WaitForParallelWorkersToAttach.

Once this function has been called, we know that all workers have
started and attached to their error queues -- so if any of them
subsequently exit uncleanly, we'll be sure to throw an ERROR promptly.
Otherwise, users of the ParallelContext machinery must be careful not
to wait forever for a worker that has failed to start. Parallel query
manages to work without needing this for reasons explained in new
comments added by this patch, but it's a useful primitive for other
parallel operations, such as the pending patch to make creating a
btree index run in parallel.

Amit Kapila, revised by me. Additional review by Peter Geoghegan.

Discussion: http://postgr.es/m/CAA4eK1+e2MzyouF5bg=OtyhDSX+=Ao=3htN=T-r_6s3gCtKFiw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9222c0d9ed9794d54fc3f5101498829eaec9e799

Modified Files
--------------
src/backend/access/transam/parallel.c | 152 +++++++++++++++++++++++++++++++--
src/backend/executor/nodeGather.c | 9 +-
src/backend/executor/nodeGatherMerge.c | 9 +-
src/include/access/parallel.h | 4 +-
4 files changed, 163 insertions(+), 11 deletions(-)