Pass extra data to bgworkers, and use this to fix parallel contexts.

 Pass extra data to bgworkers, and use this to fix parallel contexts.
 
 Up until now, the total amount of data that could be passed to a
 background worker at startup was one datum, which can be a small as
 4 bytes on some systems.  That's enough to pass a dsm_handle or an
 array index, but not much else.  Add a bgw_extra flag to the
 BackgroundWorker struct, allowing up to 128 bytes to be passed to
 a new worker on any platform.
 
 Use this to fix a problem I recently discovered with the parallel
 context machinery added in 9.5: the master assigns each worker an
 array index, and each worker subsequently assigns itself an array
 index, and there's nothing to guarantee that the two sets of indexes
 match, leading to chaos.
 
 Normally, I would not back-patch the change to add bgw_extra, since it
 is basically a feature addition.  However, since 9.5 is still in beta
 and there seems to be no other sensible way to repair the broken
 parallel context machinery, back-patch to 9.5.  Existing background
 worker code can ignore the bgw_extra field without a problem, but
 might need to be recompiled since the structure size has changed.
 
 Report and patch by me.  Review by Amit Kapila.