join: add channel_lowerts hook

[aaronmdjones]

I see the hook call being added, but nothing that reacts to it being called. What's the use-case here?

[kaniini]

That will become more obvious in future MRs. Downstream, we are refactoring the TS handling code in its entirety. This hook is the first step of making TS lowering reactive where each module and the core reacts on a TS lowering event. See the subscribers in downstream for more information, such as m_ircx_prop.c.

[aaronmdjones]

Ah, fair enough.