We one-time-pad the timestamp with a random key instead.
This will provide enough entropy to be unique, but not leak the system date.
Even with a bad RNG state it should guarantee uniqueness, however.
Fixes#17.
Nanosecond precision wasn't needed, and many legacy operating systems
don't support this POSIX.1-2001 function.
We now use plain microseconds for the timestamp, which uses the range of
the 64-bit number better as well. This will result in a Year 294247 problem.
