Handle sending failures

[federico-razzoli]

• When a client succeeds, max_failures is set to 0 and the client state is OK.

• If a client fails max_failures times, it should enter the FAIL state. The timestamp of this failure is also remembers.

• After an interval, the state changes to TRY. This interval length depends on the backoff factor.

• Only use clients that are configured and (OK or TRY).

• If the state is TRY and the client fails, it switched to FAIL, and its retry_interval is set to max(power(retry_interval, 2), max_retry_interval).

• If the state is TRY and the client succeeds, its retry_interval is set to the original value.

[federico-razzoli]

For HTTP, we're now using the Request module's backoff strategy.

When we implement this task we should stop using it, and we should implement a global backoff.