Adds exponential backoff by multiplying the `retryTimeoutMs` by
`2^attempt`. Adds a random jitter to spread out the retries. Although
in the initial implementation the retry function isn't available to the
user the code is written in a way that makes it easier to change that.

Refactors the tests to avoid using `Thread#sleep`, which is not only a
bad practice producing not deterministic tests but also makes them very
slow. A reduction of more that 90% of testing time is percieved for the
tests where this was changed.

Resolves: loki4j#194