Refactor: Connection Pooling

Nobody has claimed this yet.

This is a tracking issue for a larger effort to refactor the adapters to support connection pools. #1006 proved it could work in theory, but I think the code bass will need some more massaging to fully support it. I outlined why in https://github.com/lostisland/faraday/pull/1006#issuecomment-533371062.

## TODO

* [x] Add `#build_connection` to every adapter, which outputs the fully-configured http client from options. #1023
* [ ] Send `ConnectionOptions` to Env as `:connection` key.
* [ ] Copy settings that configure the HTTP connection from `RequestOptions` to `ConnectionOptions`: `:proxy, :bind, :timeout, :read_timeout, :open_timeout, :write_timeout`
* [ ] Teach `Adapter#build_connection` to merge `ConnectionOptions` with `RequestOptions`.
* [ ] Remove manual merging of `:proxy` into `RequestOptions`
* [ ] Teach `Adapter#initialize` to save the connection if it can:
* If _any_ of the moved `RequestOptions` settings are used, skip this!
* If the adapter supports pooling, `@pool = ConnectionPool.new(opts, &method(:build_connection))`
* If the adapter does not support pooling, `@conn = build_connection`
* [ ] Add `connection(&block)` to yield a connection for the adapter to use:
* If _any_ of the moved `RequestOptions` settings are used, yield `#build_connection`)
* If the adapter supports pooling, pass block to `@pool.with`
* If the adapter does not support pooling, yield `@conn`

## Proxy options???

https://github.com/lostisland/faraday/blob/ba7b82d9a2211c4fdd840189e246dbd7a6e4c229/lib/faraday/connection.rb#L493-L497

Start with the unchecked TODO items and the discussion in pull request #1006, then inspect the proxy-handling code linked in lib/faraday/connection.rb. Trace the completed #build_connection work in #1023 and the adapter paths it affects. Done means implementing and validating the remaining connection-option, pooling, initialization, and connection-yielding behavior, including the proxy question.

Written by the indexing model from the issue text.