Title: N^2 RDS memory for SNI

Description:
Every filter_chain_match in SNI has its own HTTP connection manager which gets its own copy of the full RDS config. For every filter chain match there's going to be a route entry (N^2).

I was looking at how to share the ConfigImpl between HTTP connection managers but what stumped me was the Server::Configuration::FactoryContext& factory_context that's passed down to ultimately be used by PerFilterConfig

1. What's the best way to address this short term? (i.e. a hack to reduce our memory footprint of 30GiB)
2. What's the real solution? (i.e. something I could contribute)
3. How does this intersect with @AndresGuedez's work?

/cc @lrouquette @bryanlatten