Replace govendor with 'go mod'

Closes #1514 (closed) #1700 (closed)

This MR preserves vendor/vendor.json in the following sense: if you run the following commands, go.mod and go.sum on this branch do not change.

# on jv-go-mod branch
rm -rf vendor go.mod go.sum
git fetch origin
git checkout origin/master vendor
go mod init gitlab.com/gitlab-org/gitaly # re-creates go.mod from vendor.json
go mod tidy # cleans up go.mod / go.sum
git reset HEAD vendor
rm -rf vendor
git status
# result: no changes

Pain points:

• will not compile on Go 1.11.3 and older
• breaks (experimental) git2go integration #1706 (closed)