New design for broker -- proxy protocol for snowflakes

This is related to the Snowflake protocol design tickets legacy/trac#29206 (moved) and legacy/trac#29293 (moved).

We want to write these protocols in a way that is not Snowflake-specific but allows any type of bridge to connect to or poll our broker/BridgeDB bridge distribution service.

The idea is that in the beginning we will start with very reliable "bridge" (which could be snowflakes) that perhaps rotate IP addresses every month or so. After that we can collect measurements and move towards more ephemeral "bridges".

Some things to keep in mind are the types of information that the snowflakes give to the broker (such as proxy version/type) to allow for metrics. This information might change so we'll want a flexible and extensible protocol.