Rework cache store back

Rework cache store back

This patch reworks the code to store data from the cache to the server,
such that the entire store loop can be replaced. The idea here is that a
platform which wishes to provide a different store loop, in particular
one which doesn't rely upon multiple data copies, may do so simply by
plugging in a new function.

Change-Id: I6573c6a5c3e93a1259266e7e1a2b7bd17357203f
Reviewed-on: http://gerrit.openafs.org/902
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>