[DyConf][Daemon] Testcases for delete, overwrite, etc.

Summary: More test case coverage.

Test Plan:

```
$ bin/gocov test github.com/gow/dyconf/daemon | tee /tmp/qqq | bin/gocov
report
ok    github.com/gow/dyconf/daemon  2.617s

github.com/gow/dyconf/daemon/server.go   httpServer.httpCallbackGet 100.00% (6/6)
github.com/gow/dyconf/daemon/server.go   sendHttpError       100.00% (3/3)
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/server.go   httpServer.start    95.00% (19/20)
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/server.go   httpServer.httpCallbackSet 81.82% (9/11)
github.com/gow/dyconf/daemon/main.go     main        0.00% (0/8)
github.com/gow/dyconf/daemon/errors.go   Error.ErrorString     0.00% (0/4)
github.com/gow/dyconf/daemon/server.go   httpServer.stop     0.00% (0/4)
github.com/gow/dyconf/daemon/errors.go   Error.MarshalJSON     0.00% (0/2)
github.com/gow/dyconf/daemon     -----------------------------   70.37% (57/81)
```