Speed up syncer shutdown

When shutting down, the syncer kernel deamon needs to iterate over all
32 of its queues, pausing for 1/4 of second each time waiting for essentially
nothing. Bump the SYNCER_SHUTDOWN_SPEEDUP so that it takes just a second to
iterate. It will still wait for another second afterwards; that bit is left
unchanged.

Reviewed By: kib
Differential Revision: https://reviews.freebsd.org/D47647