pulsesink: rework context sharing

pulsesink: rework context sharing

We also need to share the main-loop threads as this owns the context. Thus have
a class wide main-loop thread. From this we create a context per client-name.
Instead of always looking up the context, we keep this with the instance. The
reverse mapping is only needed in pulse singal handlers. This saves a lot of
locking. Also one signal handler becomes simpler as ther eis only one mainloop
to notify.

Now valgind happy - no leaks, no bad reads/writes.

This reverts major parts of commit 69a397c32f4baf07a7b2937c610f9e8f383e9ae9.

Fixes #628996