stats: Remove FakeSymbolTableImpl and related helpers

[jmarantz]

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.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[jmarantz]

/wait

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[jmarantz]

/wait

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted". Thank you for your contributions.

[mattklein123]

I guess we won't be able to clean this up until 1.16 so moving this out.

[jmarantz]

agreed; we should bake with the default being real symbol tables, but the 'fakes' being available for a release first.

[mattklein123]

@jmarantz I think we can clean all of the old code up now? WDYT?

[jmarantz]

Yes, working on it. It's easy to nuke all that code. Hopefully also easy to review.