Modify absolute dependency path to relative and Add mid config

[raymondmars]

1. Modify absolute dependency path to relatvie path , when to fork gopush-cluster project, can save project to anywhere, will not dependency origin project .
2. Add config field mid_use_long_timestamp (default is yes), let user can select long or short timestamp to use. Because redis ZADD score dose not support whole int64, so when use long timestamp as mid, it will lead mid has been changed when save private message to redis, if use short timestamp , it will be work. For some business the short timestamp as mid is enough.

[Terry-Mao]

thks，i'll reviw the code