hrpc: Add TimestampUint64 method
hrpc: Add TimestampUint64 method

The "Timestamp" method allow to set a timestamp
for a put query using the go time.Time type.
This method is taking care of converting the nanosecond to
millisecond.

This new TimestampUint64 allows the client to pass timestamp
which will not be altered.

Change-Id: Ia53cb1e7570d66d346ec3c180a3ffde5d5a9e699

tsuna/gohbase,dougklein/gohbase,dougklein/gohbase