nfs4: implement write for DcapChannel

Acked-by: Gerd Behrmann
Target: master
Require-book: no
Require-notes: no
nfs4: refactor proxy io adapter

move proxy-io common part for READ/WRITE from operation into DcapProxyIoFactory

Acked-by: Gerd Behrmann
Target: master
Require-book: no
Require-notes: no
nfs4: add support for WRITE with proxy IO

NOTICE, write is supported only with nfsv4.0.

A small code duplication will be fixes with nfs4j update.

Acked-by: Gerd Behrmann
Target: master
Require-book: no
Require-notes: no