crimson: fix dangling ClientRequest::this pointer.

crimson: fix dangling ClientRequest::this pointer.

`ShardServices::start_operation<T>()` spawns an operation,
calls `start()` on it and returns a future plus a smart
pointer controlling its life-time. Callers are responsible
to ensure the pointer doesn't go out-of-scope before
the entire execution is finished. This is error-prone.

`OSD::handle_osd_op()` forgets about its responsibility
which results in dangling `this` pointer of `ClientRequest`.
I believe the problem is much wider spread and the class
is just the tip of the iceberg.

In this commit `start_operation<T>()` is altered to extend
the life-time a bit. However, this isn't an ultimate solution
as callers are still able to e.g. put extra `this`-catching
lambdas on the returned future. A new `with_operation<T>()`-
like interface is expected as a follow-up.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>