Multi-threading support

[spuun]

This PR will make amqproxy thread-safe and ready to use with both -Dpreview_mt and upcoming Execution Contexts.

I haven't run into any deadlocks or segfaults, but if any locks is needed around Client#channel_map and/or Upstream#channels it's already prepared for by adding Client#with_channel_map and Upstream#with_channels.

[carlhoerberg]

todo: update makefile with built flags for MT

[spuun]

This needs to be refactored for crystal 1.16. Since -Dmt doesn't exist a MT context needs to be created manually (and maybe make use of an ENV to control how many threads it may use?).

[carlhoerberg]

We probably want to use Sync::Shared from https://github.com/ysbaddaden/sync instead