Ignore effectless state change requests

For state transition requests that do not change any partition
states at all, just ignore them, saving some processing time,
avoiding restarting the updater, cleaner, stopping the compactor,
commiting a new header, etc. Also avoids unncessary logging,
which makes log analysis harder and longer.

This previously used to work, before ns_server started passing
all partitions to all desired states for each state transition
call (passing partitions for cleanup that are not currently
marked as active nor passive).