Add support for remembered REST fallback host

[mattheworiordan]

See ably/docs#554

We need to add this functionality to all client libraries, but prioritise the server-side ones given they are more affected if a single DC is offline and they are publishing at a high rate.

[mattheworiordan]

Can you look at this when you have time @jdavid as it's part of our 1.1 release?

[mattheworiordan]

@jdavid any update?

[jdavid]

Will try to do this one tomorrow