Use nvim.async_call in non-main threads

[wookayin]

Due to a change from neovim python-client 0.3.0, it is now required
that nvim API calls from other threads must use nvim.async_call().

This commit wraps all such API calls from the background thread
(via self._wrap_async) to make use of nvim.async_call().

This PR attempts to fix #25.
Tested with neovim python-client 0.2.6 and 0.3.0.

[wookayin]

This is an initial attempt to fix #25. Please feel free to poke me if you see anything needs update. Some thoughts:

• Maybe proper unit test is required? (not sure)
• Should we prevent creating wrapper function instance every time?

UPD: Test is failing, will look into it.

[numirias]

Thanks for working on that!

Regarding the test failure: There are some race conditions in plugin integration tests (the ones that run a headless neovim instance) which I didn't find a clean fix for yet. The failing test you get may be one of them and you can probably safely ignore it.

Unfortunately I won't be able to verify the patch myself until tomorrow. But since the current issue makes Semshi unusable with 0.3.0, I'd prefer to merge ASAP. So let me know if you're confident it's working as intended and I'm happy to merge.

[wookayin]

I can confidently confirm that this should work with combination of py-neovim {0.2.6, 0.3.0} and NVIM {0.2.3, 0.3.1}, but whether to merge or wait a bit more is up to you.