rgw: Fix segfault due to concurrent socket use at timeout

rgw: Fix segfault due to concurrent socket use at timeout

This commit fixes a potential segfault risk when
rgw timeout handler works on the socket in one
thread while it is concurrently used by another.
The details of the fix are:

1. Instead of calling socket close(), which resets
descriptor_data in boost::asio socket and risks
segfault due to concurrent use of the socket,
the timeout handler now calls cancel() to abort
all pending ops followed by shutdown() to disable
the underlying transport. The eventual closure of
the socket will be done in the socket destructor.

2. Expose the actual boost::asio socket via get_socket()
from Connection so that the timeout handler can call
cancel() and shutdown() on it, although the socket data
member is already accessible. It allows future expansion
that wants to hide the socket even though it renders the
existing close() less useful.

Fixes: https://tracker.ceph.com/issues/58670
Signed-off-by: Yixin Jin <yjin77@yahoo.ca>