proxy mode, pt 2

The ReverseProxy code from the standard library doesn't actually
give us the control that we want. Pull it down and rip out what
we don't need, adding tests in the process.
do you think it is better to change this to a mutex?
then 
```go
t.lock()
if !available {
    t.unlock()    
    return
}
...
t.unlock()
```