libceph: osd_request_timeout option

osd_request_timeout specifies how many seconds to wait for a response
from OSDs before returning -ETIMEDOUT from an OSD request.  0 (default)
means no limit.

osd_request_timeout is osdkeepalive-precise -- in-flight requests are
swept through every osdkeepalive seconds.  With ack vs commit behaviour
gone, abort_request() is really simple.

This is based on a patch from Artur Molchanov <artur.molchanov@synesis.ru>.

Tested-by: Artur Molchanov <artur.molchanov@synesis.ru>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>