TUN-8861: Add session limiter to UDP session manager

## Summary
In order to make cloudflared behavior more predictable and
prevent an exhaustion of resources, we have decided to add
session limits that can be configured by the user. This first
commit introduces the session limiter and adds it to the UDP
handling path. For now the limiter is set to run only in
unlimited mode.

vendor/golang.org/x/tools/go/packages/doc.go

@@ -198,14 +198,6 @@ Instead, ssadump no longer requests the runtime package,
 but seeks it among the dependencies of the user-specified packages,
 and emits an error if it is not found.
 
-Overlays: The Overlay field in the Config allows providing alternate contents
-for Go source files, by providing a mapping from file path to contents.
-go/packages will pull in new imports added in overlay files when go/packages
-is run in LoadImports mode or greater.
-Overlay support for the go list driver isn't complete yet: if the file doesn't
-exist on disk, it will only be recognized in an overlay if it is a non-test file
-and the package would be reported even without the overlay.
-
 Questions & Tasks
 
 - Add GOARCH/GOOS?