Avoid leaving DataChecksumState->worker_pid to an old value

Avoid leaving DataChecksumState->worker_pid to an old value

It might be left to an old value if the launcher was terminated while
a worker was running.  launcher_exit() sends SIGTERM to the worker,
but did not clear 'worker_pid'.  Clear it, to be tidy.

Also clear it in ProcessDatabase() before starting a new datachecksums
worker, to be sure we start from a clean slate.  The codepath where
WaitForBackgroundWorkerStartup() returns BGWH_STOPPED but
worker_result != DATACHECKSUMSWORKER_SUCCESSFUL didn't clear it, while
all other codepaths did clear or set it.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/b283fbb9-298e-4953-9120-eefaf24fae20@iki.fi