This is the OSS portion of enterprise PR 2489.
This PR introduces a server-local implementation of the
proxycfg.InternalServiceDump interface that sources data from a blocking query
against the server's state store.
For simplicity, it only implements the subset of the Internal.ServiceDump RPC
handler actually used by proxycfg - as such the result type has been changed
to IndexedCheckServiceNodes to avoid confusion.
