Cache adapter connections (attempt 1)

[technoweenie]

This was an attempt to steps 2, 4, and 5 of #1024

(items copied below, since #1024 has changed)

1. ...
2. Send ConnectionOptions to Env as :connection key.
3. ...
4. Copy settings that configure the HTTP connection from RequestOptions to ConnectionOptions: :proxy, :bind, :timeout, :read_timeout, :open_timeout, :write_timeout
5. Teach Adapter#initialize to save the connection if it can:

• If any of the moved RequestOptions settings are used, skip this!
• ...
• If the adapter does not support pooling, @conn = build_connection

I started by caching the connection first, and then worrying about how to include the connection options. But, I realized I need a way to merge the connection options with request options, so that something like this will work as expected:

Faraday.new(timeout: 5, request: { write_timeout: 3 })

I think this approach was backwards. I should setup ConnectionOptions and merge it with RequestOptions at the right time, with whatever proxy changes are needed, before worrying about caching the HTTP object.

I'm keeping this PR around as an example, but I'll probably end up rewriting or cherry-picking these commits into something else later.