Support async init functions

[sfackler]

Before this PR

The server initialization function was always blocking, which meant implementors had to call off to wc.handle().block_on(...) if they needed to interact with the async runtime to e.g. spawn tasks.

After this PR

==COMMIT_MSG==
Added support for async server initialization.
==COMMIT_MSG==

Closes #270

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Description

Added support for async server initialization.

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