add svcpool_close to handle killed nfsd threads

This patch adds a new function to the server krpc called
svcpool_close(). It is similar to svcpool_destroy(), but does not free
the data structures, so that the pool can be used again.

This function is then used instead of svcpool_destroy(),
svcpool_create() when the nfsd threads are killed.

PR: 204340
Reported by: Panzura
Approved by: rmacklem
Obtained from: rmacklem
MFC after: 1 week