Added support for WouldBlock

[john-sharratt]

This patch implements a feature needed for ACME - the problem being that some certificates need to be fetched from IO engines (e.g. S3 buckets) or generated via proof of ownership challenges, however rustls uses blocking logic on the certificate resolve method and hence would lock up the thread. This is especially apparent in tokio-rustls which such blocking code would also freeze up the tokio shared thread pool.

The motivations for the choices in this patch are:

• minimize the impact to the external API, with forwards compatibility.
• do not add an async runtime to rustls itself
• minimize the amount of code changes as much as possible but while still giving the needed functionality.

The dependent library needs to be upstream before this one: https://github.com/rustls/rustls/pull/1309

[djc]

The tokio-rustls wrapper for Acceptor is called LazyConfigAcceptor, BTW.

[john-sharratt]

Excellent, thanks for the help. Closed