the plugins themselves. Add new functions to each plugin to return the
error number and given an error number, return a description.

Use it directly from  backfill plugin (instead of API call).

attempt failure (i.e. convert to event driven instead of just polling).
It does still have a 2-second timer to keep the overhead reasonable.

The plugin now gets passed the then current maximum priority to use as a
basis for setting the new job's priority (or not...).

Convert all accessors to use exclusively pointers; this should fix many of the 64-bit incompatibility problems