crimson: pass `Connection*` to Dispatch::ms_dispatch()

crimson: pass `Connection*` to Dispatch::ms_dispatch()

currently, we use a `with_gate()` in `ProtocolV2::read_message()` for
ensuring that `this` (or `Connection` holding this protocol instance)
will outlive the continuation of `dispatcher.ms_dispatch()` which
references `this->dispatcher`. but we also pass a strong reference of
connection to dispatcher. in short, we have *two* safeguards for the
same purpose.

in this change, one of these safeguards is removed -- to pass the raw
pointer of `Connection` to `Dispatch::ms_dispatch()`. the reason why
the `with_gate()` is kept is that, if we have removed `with_gate()` in
Protocol, we need to

1. let `Dispatcher::ms_dispatch()` return `void`, as it should not block
any succeeding calls.
2. add a `with_gate()` in `Dispatcher::ms_dispatch()` to ensure that
`this` is alive during the lifecycle of the continuation(s) in
`Dispatcher::ms_dispatch()`.

Signed-off-by: Yingxin Cheng <yingxincheng@gmail.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>