Make thread safety configurable

Had some free time and thought I'd give this a crack. Its very basic and should allow folks to use their preferred pooling gems. Should address: https://github.com/excon/excon/issues/330

The test is pretty rudimentary and I'm open to suggestions.
Could we just expand this in to a multi-line conditional? I think it would be a bit more consistent with code style elsewhere as well as being a bit easier to understand. Something like:

```ruby
def sockets
  if @data[:thread_safe]
    Thread.current[:_excon_sockets] ||= {}
  else
    @_excon_sockets ||= {}
  end
end
```