HPCC-8572 Std.Date functionality

Time support added to Std.Date; new timelib plugin; stringlib modifications to support time; updated tests for new Std.Date functionality.
I don't think REAL8 can be a keyed index field, and that's a very valid reason for converting the whole thing to a millisecond-based data type instead.  I'll do that, and add a Timestamp_t -> Seconds_t converter function.

Linux supports microsecond precision but tt looks like Windows supports only millisecond precision via _ftime(), and it updates the retrieved values only periodically anyway, so I think we should go for millisecond precision for both platforms.