Use default fallback hosts when custom environment set

[mattheworiordan]

When environment is set in the client SDKs, we can now rely on default fallback hosts as follows.

Fallbacks are ENV-[a-e]-fallback.ably-realtime.com. For example:

ENV-a-fallback.ably-realtime.com
ENV-b-fallback.ably-realtime.com
ENV-c-fallback.ably-realtime.com
ENV-d-fallback.ably-realtime.com
ENV-e-fallback.ably-realtime.com

Note that this change:

• It breaks a lot of tests against sandbox that assumed no fallback hosts would be used
• We need to ensure that custom hosts or custom ports result do not use default environment fallback hosts. We should only use when only the custom environment is set
• Some spec updates are needed in the spec relating to the use of fallback hosts

See related issue ably/docs#818 (no spec yet exists)

[lmars]

Just to note, we should not release this until we've run a DNS update to ensure all the fallbacks exist.

[lmars]

DNS updates have all been run so this is no longer blocked on that.

In response to points in the description:

It breaks a lot of tests against sandbox that assumed no fallback hosts would be used

Sandbox now has fallback hosts configured, in case that was the reason for the failures?

Some spec updates are needed in the spec relating to the use of fallback hosts

The spec was updated in ably/docs#965.

[SimonWoolf]

@TheSmartnik @QuintinWillison This PR was made before the spec https://github.com/ably/docs/pull/965/files was finalised. It may all be fine, but should go through that spec and check that this PR implements the final spec correctly

[jamienewcomb]

@TheSmartnik did you managed to look at Simons comment
#196 (comment)

If you are happy could you approve and we will get this merged

Thanks

[TheSmartnik]

@jamienewcomb sorry, I've missed the comment. I'll double-check it later today and reply

[lmars]

@TheSmartnik it would be great to get this merged, did you get chance to review Simon's comment?