Go raises the "max open files" soft limit to match the hard limit for
itself, but has special logic to reset the original soft limit before
forking a child process. This logic does not apply if the file limit is
set explicitly. Add a pair of Getrlimit / Setrlimit calls so that we
(1) preserve the default Go limit behavior for ourselves, and
(2) keep these same limits when launching Envoy.
* support loading route configuration via rds
* fix any shadowing
* fix test
* add fully static option
* support dynamically defined rds
* fix build
* downgrade opa
