Incremental backoff and jitter

[maratal]

Closes #1431
There is an unresolved conversation in the reference implementation for channel - ably/ably-js#1008 (comment) I've decided to take state check from Owen's implementation.

[owenpearson]

The implementation LGTM, although I don't know much objective-c so it's hard to say for sure.

A couple of things to mention:

• There are some newly skipped tests here which shouldn't be the case - is there a particular reason why these tests can't be fixed?
• It might be worth considering using a seed for random number generation in the tests here to make retry intervals deterministic (this wasn't possible in ably-js because Math.random() doesn't take a seed argument).

[lawrence-forooghian]

Will review the content of the PR next, but re the skipped tests, I think ideally we should investigate and fix this here, and not leave it for #1526 and #1527. The newly-skipped tests are tests that were passing before, and have only started to fail after we added these new tests. Is there something that makes you think that the problem lies with the existing tests, and not the new ones that we've added?

[maratal]

I guess most of all other failing/flaky tests were reported like that, but I agree we should stop this practice and try to fix it in place.

[lawrence-forooghian]

Are the tests that you've marked as skipped failing consistently after adding the new tests?

[lawrence-forooghian]

I need to dive into this properly and understand the tests, but here are some initial thoughts from looking over the code.

[lawrence-forooghian]

I just tried (locally) un-skipping the tests mentioned in #1526 and #1527, and I didn't see them failing. But what I did see was some other failures:

• RealtimeClientConnectionTests.swift:2193 test__058__Connection__connection_request_fails__connection_attempt_should_fail_if_not_connected_within_the_default_realtime_request_timeout(): Waited more than 20.0 seconds
• RealtimeClientConnectionTests.swift:2206 test__058__Connection__connection_request_fails__connection_attempt_should_fail_if_not_connected_within_the_default_realtime_request_timeout(): Start date or end date are empty
• RealtimeClientConnectionTests.swift:2246 test__059__Connection__connection_request_fails__if_connection_attempt_fails_connection_state_will_transition_to_DISCONNECTED_with_periodic_connection_attempts_with_incremental_backoff_and_jitter_until_SUSPENDED(): expected to be close to <1.0933> (within 0.2187), got <1.3333>

[lawrence-forooghian]

I'm going to close this PR — I intend to land the remainder of this work in #1611, #1612, #1613.