remove 'static lifetime bound on http1/2 client IO

[jgraef]

The method hyper::client::conn::http1::Builder::handshake has a 'static lifetime bound on the type parameter T (the underlying connection). This lifetime bound is not necessary and prohibits users of the library to borrowing connections to a hyper client.

There are similar bounds on the Connection struct, handshake method, plus all those for http2.

This PR removes these 'static lifetime bounds.

[jgraef]

There's at least one 'static that I missed1. I will update this PR soon.

There are also some Unpin trait bounds that are not necessary. Should I open a new PR for them?

[dswij]

Probably related: #3595

I don't see any problem with this, unless I'm missing something? @seanmonstar

[jgraef]

Should I update this PR with that one more 'static bound removed? I can also bring it up-to-date with master. Do you prefer rebase or merge?

[seanmonstar]

Rebase or new commits, whichever you find easier to push. We can always squash when merging.