libceph: encapsulate connection kvec operations

Encapsulate the operation of adding a new chunk of data to the next
open slot in a ceph_connection's out_kvec array.  Also add a "reset"
operation to make subsequent add operations start at the beginning
of the array again.

Use these routines throughout, avoiding duplicate code and ensuring
all calls are handled consistently.

Signed-off-by: Alex Elder <elder@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>