Le big rewrite

[lipanski]

A rewrite using async and Hyper with full support for multi-threaded test runs, multiple servers/endpoints and other goodies.

The old interface will still be accessible in this version, but will be marked as deprecated, since it still relies on the locking mechanism.

Closes https://github.com/lipanski/mockito/issues/92 https://github.com/lipanski/mockito/issues/94 https://github.com/lipanski/mockito/issues/144 https://github.com/lipanski/mockito/issues/150

Potentially closes https://github.com/lipanski/mockito/issues/111 https://github.com/lipanski/mockito/issues/140 https://github.com/lipanski/mockito/issues/147

TODO:

• [x] Document the server
• [x] Add an HTTP2 test
• [x] Introduce an Error type
• [x] Check for unnecessary unwraps
• [x] Update the README
• [x] Update/remove the benchmark test
• [x] Introduce an error context