add handle() to return zhangle_t, and add output parameter result path for CreateBuilderImpl::forPath

[whunmr]

1. add handle() to return zhangle_t, so user can call zoo_* functions exposed by zookeeper.h, such as zoo_client_id.

2. and add output parameter result path for CreateBuilderImpl::forPath so user can create ZOO_SEQUENCE node, and get the result znode path.