pool: Account#allocateNow() should trigger sweeper to get some space

Currently, Accoutn#allocateNow() utilize only free space, but does not
uses reclaimable space. As a result allocateNow will fail on pools with
all files cached.

This change makes sure, that sweeper get informed about the requested space.

Acked-by: Gerd Behrmann
Target: master
Require-book: no
Require-notes: no
pool: add Allocator#allocateNow()

add a non-blocking version of allocate.

Acked-by: Gerd Behrmann
Target: master
Require-book: no
Require-notes: no
pool: add soft/hard space allocation modes

The classic dcache behavior it to block if there is no space available to write requests.
This strategy is not always matches clients expectations.

Introduce SOFT/HARD allocation modes for mover channel.
Update movers to use new option.
Introduce NoSpaceLeftException
Updated nfs mover to use soft allocation and return NO_SPACE if allocation failed

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