[PATCH] Fix for infinite signal loop in parallel scan

Attached is the patch we are fully testing at Adjust.  There are a few non-obvious aspects of the code around where the patch hits.    I have run make check on Linux and MacOS, and make check-world on Linux (check-world fails on MacOS on all versions and all branches due to ecpg failures).  Automatic tests are difficult because it is to a rare race condition which is difficult (or possibly impossible) to automatically create.  Our current approach testing is to force the issue using a debugger.  Any ideas on how to reproduce automatically are appreciated but even on our heavily loaded systems this seems to be a very small portion of queries that hit this case (meaning the issue happens about once a week for us).

The main problem is that under certain rare circumstances we see recovery conflicts going into loops sending sigusr1 to the parallel query which retries a posix_falloc call.  The call gets interrupted by the signal perpetually and the replication cannot proceed.

The patch attempts to handle cases where we are trying to cancel the query or terminate the process in the same way we would handle an ENOSPC in the resize operation, namely to break off the loop, do relevant cleanup, and then throw relevant exceptions.

There is a very serious complication in this area, however, which is that dsm_impl_op takes an elevel parameter which determines whether or not it is safe to throw errors.  This elevel is then passed to ereport inside the function, and this function also calls the resize operation itself.  Since this is not safe with CHECK_FOR_INTERRUPTS(), I conditionally do this only if elevel is greater or equal to ERROR.  

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