os/kstore: cache in-flight stripe updates in memory

os/kstore: cache in-flight stripe updates in memory

The write process may do a read/modify/write on a stripe.  In order to
allow multiple writes to coexist within the same transaction, we need to
be able to "see" our writes.

Clear the "cached" stripe values when the last TransContext touching an
onode is finished.  In theory we could pin memory with a constant stream
of updates to an object; we may need to address that later.

Signed-off-by: Sage Weil <sage@redhat.com>