nfs-client: add sendCompoundInSession

Motivation:
Some NFSv4.1 operations must be bound to a session, some are not. Thus
for some compound requests the first operations must be SEQUENCE, that
specifies the session id, slot to use and sequence.

Modification:
introduce sendCompoundInSession method that takes care about session
bookkeeping.

Result:
Automatic session processing when the corresponding sendCompound is
used.

Acked-by: Paul Millar
Target: master