Track time internally using 64-bit microseconds.

Avoids strangeness with scheduling the next session to send a packet on
when the server's elapsed time wraps around the 32-bit microsecond mark
(every ~71 minutes).  We still transmit 32-bit timestamps on the wire,
the wrapping doesn't cause problems there.

Change-Id: I29a7d8158f0ae66597f9e9a149f61e7265f03150
3 files changed