s3: printing: Fix problem with server taking too long to respond to a MSG_PRINTER_DRV...

s3: printing: Fix problem with server taking too long to respond to a MSG_PRINTER_DRVUPGRADE message.

Receiving a MSG_PRINTER_DRVUPGRADE causes
smbd to iterate over all printers looking
for ones that uses the driver. This is a very
expensive operation requiring a read of all
registry printer parameters.

On a system with a large number of printers,
this causes the clients to timeout (smbd
can take longer than 60 seconds to respond).

This patch fixes the problem by forwarding
the MSG_PRINTER_DRVUPGRADE to the background
lpq queue updater process and allowing it to
take care of the updating of the changeid
in the registry, allowing the smbd connected
to the client to return to processing requests
immediately.

https://bugzilla.samba.org/show_bug.cgi?id=9942

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Tue Feb 18 17:48:30 CET 2014 on sn-devel-104