Mediate access to mock server with a mutex

[Diggsey]

The docs currently recommend running tests with --test-threads=1, but this means that none of the tests can be run in parallel.

A simple solution without changing the design would be to take a global mutex whenever the first mock end-point is created on a thread, and release that mutex whenever the last mock end-point is dropped.

[lipanski]

it's not solving the core problem but it could allow tests not using mocks to perform in parallel, which is neat. would you be interested in making a PR?

[lipanski]

Closed by #35
@Diggsey please let me know if this works for you. Also, if it happens that you're using mocks inside doctests, please check #36