Previously, there was no notification when capacity is made available by
requests completing. This patch fixes the bug.
This also switches the tests to use `MockTask` from tokio-test.
I've implemented `std::error::Error` for the error types in the `tower-balance`, `tower-buffer`, `tower-in-flight-limit`, and `tower-reconnect` middleware crates.
This is required upstream for runconduit/conduit#442, and also just generally seems like the right thing to do as a library.
Provides a middleware that sets a maximum number of requests that can be
in-flight for the service. A request is defined to be in-flight from the
time `call` is invoked to the time the returned response future
resolves.
This maximum is enforced across all clones of the service instance.
