In gimp-parallel, always flush the async-operations queue (by
executing all remaining operations on the caller thread) when
setting the async-pool thread count to 0 (as happens when setting
GEGL_THREADS=1, per the previous commit,) and not only when
shutting GIMP down.  Otherwise, pending asynchronous operations
can "get lost" when setting GEGL_THREADS to 1.

Additionally, in gimp_gegl_init(), initialize gimp-parallel before
before connecting to GimpGeglConfig's "notify::num-processors"
signal, so that the number of async threads is set *before*
GEGL_THREADS, in order to avoid setting GEGL_THREADS to 1 while
async operations are still executing.

Also, allow setting the number of gimp-parallel-distribute threads
while a gimp-parallel-distribute function is running (which can
happen if gimp-parallel-distribute is used in an async operation,
as is the case for histogram calculation), by waiting for the
parallel-distribute function to finish before setting the number of
threads.