This matches with libpq's behavior, which doesn't allow arbitrary
parameters. You can still set arbitrary parameters, just through the
`options` field.
* `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.
