[DyConf][Daemon] More test cases coverage and some code cleanup

Summary:
Added more cases to cover more cases. Also reduced the time interval
between testcases.

Test Plan:

```
$ bin/gocov test github.com/gow/dyconf/daemon | tee /tmp/qqq | bin/gocov
report
ok    github.com/gow/dyconf/daemon  0.721s

github.com/gow/dyconf/daemon/server.go   httpServer.httpCallbackSet 100.00% (14/14)
github.com/gow/dyconf/daemon/server.go   httpServer.httpCallbackGet 100.00% (6/6)
github.com/gow/dyconf/daemon/errors.go   Error.GetErrorString    100.00% (5/5)
github.com/gow/dyconf/daemon/server.go   httpServer.stop     100.00% (4/4)
github.com/gow/dyconf/daemon/server.go   sendHttpError       100.00% (4/4)
github.com/gow/dyconf/daemon/server.go   sendHttpJSONResponse    100.00% (2/2)
github.com/gow/dyconf/daemon/daemon.go   dyconfDaemon.stop     100.00% (1/1)
github.com/gow/dyconf/daemon/errors.go   Error.GetErrorNo    100.00% (1/1)
github.com/gow/dyconf/daemon/server.go   httpServer.start    90.00% (18/20)
github.com/gow/dyconf/daemon/server.go   httpServer.httpCallbackDelete 85.71% (6/7)
github.com/gow/dyconf/daemon/daemon.go   dyconfDaemon.init     84.62% (11/13)
github.com/gow/dyconf/daemon/main.go   main        0.00% (0/8)
github.com/gow/dyconf/daemon/errors.go   Error.Error       0.00% (0/1)
github.com/gow/dyconf/daemon     -----------------------------   83.72% (72/86)
```