Change Traffic Ops middleware wrapper symbols to be public (#4171)

* Change TO middleware wrapper symbols to be public

The primary goal of this is to make them accessible to plugins,
which will almost always want to add the access control, checksum,
server, and gzip headers.

To that end, this also includes an example plugin using middleware.

* GoDoc TO middleware; move rfc, util funcs to lib

Fully GoDoc's traffic_ops/traffic_ops_golang/routing/middleware.

Also moves some RFC and utility symbols to lib/go-rfc and lib/go-util.

This also meant moving some RFC symbols from go-tc to go-rfc, since
go-rfc doesn't and can't depend on go-tc.
56 files changed