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

[kinto0]

sorry about delay in reviewing! please bother me if this happens again

looks great. thanks for the changes

[meta-codesync]

@kinto0 has imported this pull request. If you are a Meta employee, you can view this in D84510914.

[stroxler]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@kinto0 merged this pull request in 66b3240.