Add new time sliced overlay survey

[bboston7]

Description

Resolves #4169, #4164. Partially resolves #2592 (script update remains).

This pull request adds a new survey mode that aims to be more deterministic and easier to compare across runs. It also adds some additional fields to the survey schema. It is very faithful to the design outlined in the Unified Network Survey Work Proposal.

While this PR includes all of the new functionality, it is still missing a few things I'd like to do before it's completely done:

• [x] Document new schema
• [x] Document new commands / how to run survey
• [x] Document new metrics in metrics.md
• [x] Change tests to use HTTP endpoints instead of calling into SurveyManager
• [x] Deduplicate shared code between tests
• [x] Update overlay survey script (or open an issue and update this script separately)
  • Opened #4302 to track this. This PR is huge as is and the script changes don't need to land with the release.
• [ ] Merge https://github.com/stellar/stellar-xdr/pull/181 and update XDR submodule(s) to point to that instead of my own branch.

That being said, this code is ready for a first set of review!

NOTE: I'd expect any next builds to fail as I only updated the curr branch with my XDR changes (see https://github.com/stellar/stellar-xdr/pull/181). I'm not sure what the protocol is for updating XDR.

Checklist

• [x] Reviewed the contributing document
• [x] Rebased on top of master (no merge commits)
• [x] Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• [x] Compiles
• [x] Ran all tests
• [ ] If change impacts performance, include supporting evidence per the performance document

[bboston7]

I just squashed and rebased this on top of master to pull in the new overlay changes.

[bboston7]

I've rebased and squashed this down to a single commit in preparation for merging!

[marta-lokhova]

r+ 075df81d36699aaf1785c4b55b62d863862e14bb