Revert "swapping to tm_timestamp to avoid TC wrap"

See my comment on the PR for why there was no issue with integer wrap: https://github.com/tessel/firmware/pull/88#discussion_r18419604.

This adds an issue, however: if this is made asynchronous and the time is set while a C3000 operation is pending, the time will jump forward and the timeout will happen early. For things like timeouts, you want a monotonic time, not wall clock time. Since firmware has no reason to care about wall clock time, any use of `tm_timestamp` in firmware is suspect. Firmware shouldn't be using floating point, either.