Add support for TSP snapshot request

[rchiodo]

This PR adds another request handler for TSP = "typeServer/getSnapshot". The snapshot request is how the client side of a TSP connection indicates which 'state of the world' it's currently querying about. The server side keeps a 'snapshot' to indicate when the state of the world has changed.

Internally I used the 'epoch' and the 'RecheckFinished' event to indicate what the snapshot is and when it's changed.

[connernilsen]

Hey @rchiodo, thanks for doing this. Kyle's out until Friday, and I don't know as much of what's going on, so I'm going to assign to Kyle for now and let him review when he's back. Let me know if there's anything else you need in the meantime :)

[rchiodo]

Hey @rchiodo, thanks for doing this. Kyle's out until Friday, and I don't know as much of what's going on, so I'm going to assign to Kyle for now and let him review when he's back. Let me know if there's anything else you need in the meantime :)

Thanks for letting me know :). No rush but I was worried Meta was no longer interested in TSP.

[kinto0]

looks great, nice thought on the RecheckFinished event. just a few suggestions, mostly to do with the test

[rchiodo]

Needed this to process so that async events are processed in the tsp server (sometimes RecheckEvent is async). Seemed like DidChangeWatchedFiles notification causes an async event.