add thread-safe dispatcher callback mechanism

[arnetheduck]

callSoon currently is well-defined for the current thread but cannot be called from a different thread.

ThreadSignalPtr can be used to overcome this limitation but comes with the significant downside that each signal requires its own file descriptor / handle - while this approach works for specific cases it becomes costly in a general-purpose setting such as https://github.com/status-im/nim-async-channels where each channel ends up needing its own instance.

It would in theory be possible to use a thread-local ThreadSignalPtr for channels in particular, but this would still not achieve the higher goal of sharing a single wake-up / notification mechanism per event loop instance.

A thread-safe callSoon allows threads to post work to the dispatcher of another thread and getting called back from within the scope of that thread "soon" - for example, in the AsyncChannel when a reader appears and the channel is empty, the reader can register its own dispatcher with the channel so that when a queue item appears, the channel can wake up the corresponding dispatcher and complete the future from within the correct thread-local context.

A simple unbounded MPSC queue is used for storing cross-thread callbacks - the cost is roughly equivalent to the existing AsyncCallback when used with a closure environment, ie one allocation per posted work item plus whatever the caller needs.

The design does not depend on the specific queue being used, ie a future version could use a ring buffer + a spill queue or similar designs that reduce allocations.

Coupled with the queue is a cross-platform "wakeup" mechanism similar to the one used in ThreadSignalPtr - when a callback is posted to the queue, we also make sure to wake up the dispatcher so that indeed the callback happens "soon".