admin/info: Add HTTPStats value as part of serverInfo() struct. (#4049)

Remove our counter implementation instead use atomic external
package which supports more types and methods.

vendor/go.uber.org/atomic/glide.yaml

@@ -0,0 +1,6 @@
+package: go.uber.org/atomic
+testImport:
+- package: github.com/stretchr/testify
+  subpackages:
+  - assert
+  - require