Refactor metrictank repo to be compatible with go modules

[ywwg]

This PR refactors the entire metrictank repository to make it compatible with Go modules as well with vscode's static compilation. All of the Makefile commands have been fixed (as far as I know). I believe travis/circleci is also working.

As part of the change, the vendor/ directory has been removed from checked-in source, instead users should run go mod vendor locally as needed.

[replay]

I think to fix the pointed out search & replace issue the easiest would be to just search and replace these strings:

github.com/grafana/metrictank/pkg/blob github.com/grafana/metrictank/pkg/issue(s) github.com/grafana/metrictank/pkg/pull(s)

and remove the pkg again from them

[ywwg]

ok I rolled the deps back that I could get from gopkg.lock, but unfortunately there are still massive diffs between the checked-in vendor/ dir in metrictank and the versions created by go mod vendor. What's more, it looks like the checked-in vendor/ dir has been hacked up, removing README and CHANGES files, making it impossible to know which version was downloaded. And given the huge number for dependencies I'm not sure there's a reliable way to pin all the deps at the correct old versions.

I think the next step is figuring out a way to test this branch to see if any of the module updates break anything.

[ywwg]

due to the risk of instability, we are going to put off merging this until ~Q1 or so.