Non-breaking named mocks

Reworks #40 by @MarcoIeni to make it a backwards-compatible change. It cuts a new release - 0.4.4.

Other changes that do not impact the public API:

adds a Display implementation for Times
falls back to the mock number if no name has been specified by the user (e.g. Mock #1 failed);
rename MockSet to ActiveMockSet,
upgrade tokio, reqwest and hyper to their latest releases.

Closes #45 Closes #46

LukeMathWalker / wiremock-rs