Private trait in Service implementation bounds for Connect

[mzabaluev]

The implementation of Service for Connect features a type parameter bound of ConnectExecutor, which is a private trait (leading to a rabbit hole of prerequisites and parameter bounds). This makes it practically impossible to use Connect with a generic executor, as the required bound for the executor type is an internal detail of tower-hyper.

[mzabaluev]

It looks like the executor bound could be respecified in terms of TypedExecutor and hyper::client::conn::Connection, but the error-eating MapErr looks a bit temp-ish to export. Does the implementation simply need more principled error propagation before the bounds can be stabilized?

[mzabaluev]

tower-h2 has client::Background to represent the background task generically. It would be nice to have a similar wrapper type. Perhaps it could later get a customizable error handling type hook as an alternative to internal logging (discussed in #45).