Improve performance of string from date creation.

[nlutsenko]

This improves creation of string from dates by ~5 times, by using sqlite directly instead of generic NSDateFormatter.
As a result of this improvement:

• 5-10% win in LDS performance on pinning (I am serious, there is a huge ton of time spent here)
• huge performance gain on encoding any JSON that has our dates in it, say for any network request

Contributes to #32, as it improves LDS perf.

Now, onto notes - I intentionally am not resetting the binding on the statements,
as there is slight performance gain there (about 5% compared to resetting them every time).
Couldn't come up with implications on not doing this, as I feel it's safe to do it in this use-case.

[richardjrossiii]

Resetting is safe to not do because we're only binding primitives (aka double here). Those will be directly copied into the statement's internal buffer. Love it!