Remove st_idle variable, duplicating st_xprt.

MFC after:	2 weeks

(cherry picked from commit c73f36ad0d93d806a2c6cb1231ea07d1924ed08e)
(cherry picked from commit 3effbd6f22ddda1027881ce6623c803ba260f8ac)
Split RPC pool threads into number of smaller semi-isolated groups.

Old design with unified thread pool was good from the point of thread
utilization.  But single pool-wide mutex became huge congestion point
for systems with many CPUs.  To reduce the congestion create several
thread groups within a pool (one group for every 6 CPUs and 12 threads),
each group with own mutex.  Each connection during its registration is
assigned to one of the groups in round-robin fashion.  File affinify
code may still move requests between the groups, but otherwise groups
are self-contained.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.

(cherry picked from commit 18c81a620b48a1fe1d86fe9f1ba4dc0fc06024aa)
(cherry picked from commit 2c8c1b205b7f5acc990cc9033affc262ffdd8bf9)
Fix race in r267221.

MFC after:	2 weeks

(cherry picked from commit 9264ec94fea14775eae6ce1ea9f982fb3a25b81e)
(cherry picked from commit 14a9ec74fc7cd8db5b62367dd713f6928e239b0e)