The "socks" feature has been removed for a while now, the optional
dependency on the "socks" crate commented out.
The code for actually providing the socks feature was, however, still
mostly present, if a bit out of date.
This commit re-adds the socks feature using the tokio-socks (instead of
socks) crate.
Closes#620
To allow for the default-tls to change to a different backend by
default, this adds a new `native-tls` optional feature. Any TLS feature
that was only available using native-tls now requires the `native-tls`
feature to be enabled.
