[DyConf][Daemon] Fix panics caused due to multiple http handle registration

Summary: We can run multiple test cases for daemon now. Running more than one test casue used to cause panics due to re-registering of handler functions to DefaultServeMux. This diff clears DefaultServeMux after the test case to avoid these panics.

Test Plan:
  $ bin/gocov test github.com/gow/dyconf/daemon | tee /tmp/qqq | bin/gocov report
  ok    github.com/gow/dyconf/daemon  1.051s

  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/daemon.go   dyconfDaemon.init     84.62% (11/13)
  github.com/gow/dyconf/daemon/server.go   httpServer.httpCallbackGet  83.33% (5/6)
  github.com/gow/dyconf/daemon/server.go   httpServer.start    80.00% (16/20)
  github.com/gow/dyconf/daemon/server.go   httpServer.httpCallbackSet  63.64% (7/11)
  github.com/gow/dyconf/daemon/main.go   main        0.00% (0/8)
  github.com/gow/dyconf/daemon/server.go   httpServer.httpCallbackDelete   0.00% (0/7)
  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     -----------------------------   55.56% (45/81)

Task: T9

Reviewers: tommy.testy

Reviewed By: tommy.testy

Maniphest Tasks: T9

Differential Revision: http://dev.chetan.io/D42