crimson/osd: wait for SIGINT and SIGTERM before stopping

crimson/osd: wait for SIGINT and SIGTERM before stopping

this change addresses an regression introduced by
37b83f4ed7ca69f105b93bf482cb2289cbaf9a4d. as we should not stop
services without being asked to do so.

in this change, signal handler for SIGINT and SIGTERM is registered to
handle these signals, and in the seastar thread, we wait until any of
these two signals is caught.

Signed-off-by: Kefu Chai <kchai@redhat.com>