Replace time.Time with uint64 in coinswap msgs

[zhangyelong]

In order to be compatible with different language versions of the clients, we need to use uint64 to receive deadline in coinswap msgs instead of time.Time.

[zhangyelong]

Hmm, seems int64 is better
Because the system uses int64 for Unix Time conversion, Unix Time itself can be negative, representing the time before 1970. Although we will never use it, using int64 can prevent us from doing another meaningless conversion.