Add process wide concurrency protection to Telepresence cache files.

[thallgren]

The Telepresence cache files are accessed by multiple processes without proper concurrency protection to prevent corruption. This is bad. It makes our CI-tests unstable, because they often do things in rapid succession that normal users wouldn't, but it may also cause problems in other scenarios as well, such as using concurrent containerized daemons, scripts, or intercept specs.

We must implement proper file locking. Pending this proposal to make the internal lockedfile Go package public, we can use the rogpeppe/go-internal/lockedfile.