MFC r267221, r267278:
Introduce new per-thread lock to protect the list of requests.

This allows to slightly simplify svc_run_internal() code: if we processed
all the requests in a queue, then we know that new one will not appear.
MFC r267223:
Remove st_idle variable, duplicating st_xprt.
MFC r267228:
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 r267232, r267239:
Use atomics to modify numvnodes variable.

This allows to mostly avoid lock usage in getnewvnode_[drop_]reserve(),
that reduces number of global vnode_free_list_mtx mutex acquisitions
from 4 to 2 per NFS request on ZFS, improving SMP scalability.
MFC r267362:
Remove unneeded mountlist_mtx acquisition from sync_fsync().

All struct mount fields accessed by sync_fsync() are protected by MNT_MTX.
MFC r267351:
Move root_mount_hold() functionality to separate mutex.

It has nothing to share with mutex protecting list of mounted file systems.
MFC r267392:
Implement simple direct-mapped cache for popular filesystem identifiers to
avoid congestion on global mountlist_mtx mutex in vfs_busyfs(), while
traversing through the list of mount points.

This change significantly improves NFS server scalability, since it had
to do this translation for every request, and the global lock becomes quite
congested.

This code is more optimized for relatively small number of mount points.
On systems with hundreds of active mount points this simple cache may have
many collisions.  But the original traversal code in that case should also
behave much worse, so we are not loosing much.
MFC r267479:
Fix/improve fhe_stats sysctl output.