common/async: Update `use_blocked` for newer asio

common/async: Update `use_blocked` for newer asio

Reimplement with `initiate` rather than the old style. This
necessitates getting rid of the old `async::Completion` in anything
that was calling it, and other changes.

Also, use disposition for error handling.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
(cherry picked from commit e81d4eae4e76f9c279cd8f146dd6cc132a5ed51a)

Conflicts:
	src/rgw/driver/rados/rgw_datalog.cc
 - This commit was on top of another bugfix with changes to
   startup/shutdown that has not yet merged.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>