mds: improve eviction usability and update docs

mds: improve eviction usability and update docs

The "session ls" and "session evict" are now
"client ls" and "client evict" (the old ones
are still there for backwards compatibility).

The automatic client eviction now emits
cluster logs that call the client by its
friendly name (usually the hostname).

Signed-off-by: John Spray <john.spray@redhat.com>