Fix status section out of order problem

mutiple status.go in go/cmd add status section in vt status server
for system monitoring purpose. However, the problem is they use init
function and may cause potential section out of order problem on
status page if there is another place to also add a new section.

The fix is to introduce a pluggable hook in each status.go so that
whenever another place want to add a new section, it could do so by
assign the hook in its init method.
change dbconfigs API to accept specific credentials

1. Add a DBConfigFlag bitmask param to RegisterFlags and
   Init methods
2. Add a return value for RegisterFlags method to indicate
   registered DBConfigFlag
3. Add dbconfigs test
Merge pull request #242 from youtube/replication

Replication
Merge pull request #234 from yaoshengzhe/fix_dbconfig_api_with_bitmask

change dbconfigs API to accept specific credentials
Merge pull request #230 from yaoshengzhe/fix_status_racing_condition

Fix status section out of order problem