revproxy: rename lk to mu (from httputil)
revproxy: rename lk to mu (from httputil)

Ported from Go's e55896b9.

The conventional name for a sync.Mutex is "mu".

These "lk" names date back to a time before conventions.

lonelycode/tyk,lonelycode/tyk,mvdan/tyk,mvdan/tyk,mvdan/tyk,mvdan/tyk,mvdan/tyk,mvdan/tyk,mvdan/tyk,lonelycode/tyk,mvdan/tyk