The goal of FakeSymbolTableImpl is to put the SymbolTable API in place. That will enable us to incrementally transform all places in the Envoy codebase where stat-names are constructed to pre-allocate at startup all StatNames constituent strings.

Once the real Symbol TableImpl is in place, encoding strings into symbols requires taking a symbol-table lock, so we don't want to do that in the hot-path.

So FakeSymbolTableImpl allows us to deploy the Symbol API without worrying about lock contention.

Once there are no more hot-path encoding of strings into SymbolTable, we can swap in SymbolTableImpl and delete FakeSymbolTableImpl.