Making getting the current time faster

This sucks:
https://github.com/bitemyapp/blacktip/blob/master/src/Database/Blacktip.hs#L45

Worse, it sucks in multiple ways.

First:

`(round . (*1000))`

`round` is slow-as. The `(*1000)` ideally wouldn't even be necessary.

Second:

`PSX.getPOSIXTime` originates in a syscall and goes through like 3 redundant transformations before I get anything out of it. This is slow and unnecessary! Ideally we'd want a millisecond-resolution with the same 64-bit representation in as little work as is possible.

Worth noting that getPOSIXTime doesn't guarantee monotonicity. I don't know how likely of a problem that is.