JBAS-769 Clear holds shelf cache poll

JBAS-769 Clear holds shelf cache poll

When called with a new API name, the clear-shelf process returns the
cache key first thing to the caller (via respond_complete).  Instead
of waiting for the clear-shelf API call to complete, which often times
out, the client polls the server with the cache key at regular intervals
to see if the print data has been cached.  Once found, printing is
initiated.

Signed-off-by: Bill Erickson <berickxx@gmail.com>