Add support for specifying timestamp precision using standard modifiers

[Mytherin]

This PR adds support for specfiying TIMESTAMP precision using the standard type modifier syntax, e.g.:

CREATE TABLE ts_precision(
	sec TIMESTAMP(0),
	msec TIMESTAMP(3),
	micros TIMESTAMP(6),
	nanos TIMESTAMP (9)
);

We only support seconds, milliseconds, microseconds and nanoseconds - i.e. 0, 3, 6 and 9. Other modifiers are rounded up, i.e. TIMESTAMP(1) will be turned into TIMESTAMP(3) (ms).

Only modifiers until nanoseconds (9) are supported. Higher modifiers will throw an exception.