nfs: do not create an instance of StripingPattern per pool

as we do not support different strategies, instantiated only once.

Acked-by: Karsten Schwank
Target: master
Require-book: no
Require-notes: no
nfs: remove obsolete comment

as we already sending the complete list of addresses.

Acked-by: Karsten Schwank
Target: master
Require-book: no
Require-notes: no
nfs: introduce PoolDeviceMap for pool to deviceid mapping

Acked-by: Karsten Schwank
Target: master
Require-book: no
Require-notes: no