replace dep with go mod (#3907)

Replace sha1sum with jack's gosum and get rid of
vendor-deps.
Also don't compute hash on vendor/ contents.
Instead hash go.sum.

Disable unconvert lint check. It does not
work very well with go mod.

Remove update_vendor_deps once and for all.

Upgrade to go 1.12

Closes: #3919 #3630

tools/gometalinter.json

@@ -2,7 +2,7 @@
     "Linters": {
         "vet": "go tool vet -composites=false :PATH:LINE:MESSAGE"
    },
-    "Enable": ["golint", "vet", "ineffassign", "unconvert", "misspell"],
+    "Enable": ["golint", "vet", "ineffassign", "misspell"],
     "Deadline": "500s",
     "Vendor": true,
     "Cyclo": 11,