Go migration follow-ups

[jamespfennell]

I will shortly be merging the Go migration into mainline. This issue is to track outstanding work - typically features from the Python code that weren't added to the Go code before the merge.

• Geographic search (Slow queries when using POST /stops endpoint for large distances #85)
• Feed update garbage collection (Investigate if feed update trimming task needs to be optimized #58)
• Feed flush tool
• Check if the SF BART still works - I suspect it won't, because it requires logic to map static data to realtime data.
• System transfers feature
• Transit topological sort (Split into components -> transitive reduction -> tree-graph-tree decomposition -> 3 x sort)