* `Connection` is now parameterized over the stream type, which can be
any `AsyncRead + AsyncWrite`.
* The `TlsMode` enum is now a trait, and `NoTls`, `PreferTls`, and
`RequireTls` are types implementing that trait.
* The `TlsConnect` trait no longer involves trait objects, and returns
channel binding info alongside the stream type rather than requiring
the stream to implement an additional trait.
* The `connect` free function and `ConnectParams` type is gone in favor
of a `Builder` type. It takes a pre-connected stream rather than
automatically opening a TCP or Unix socket connection.
Notably, we no longer have any dependency on the Tokio runtime. We do
use the `tokio-codec` and `tokio-io` crates, but those don't actually
depend on mio/tokio-reactor/etc. This means we can work with other
futures-based networking stacks.
We will almost certainly add back a convenience API that offers
something akin to the old logic to open a TCP/Unix connection
automatically but that will be worked out in a follow up PR.
Also add the version number to the feature name. This both makes it more
obvious when using them and opens the window for support of multiple
versions simultaneously in the future.
