- Move Spring Cloud Stream classes logic into `static` methods
to avoid eager load for those classes which are not present on CP
- Remove `emptyMessage()` since the contract for `ChannelInterceptor`
never accept a `null`
- Remove thread state manipulation from the `postReceive()` since
no one takes care about thread local afterwards
- Remove `afterReceiveCompletion()` in favor of its `default` impl
in the interface: we don't do thread local manipulation for `receive()`
since `afterReceiveCompletion()` is called *before* the message is really
returned to the target subscriber on the channel

Even if we don't need a thread local store, we still
need to call `span.end()` and set an exception tag
to it if any.
So, just reuse an existing API which includes thread local.
Even if we don't need it logically, there might be some
other interceptors in between which would like to take a
span from thread local do something even on that "void"
`receive()` operation