CDRIVER-423 ISO-8601 string or $numberLong dates

Provide support for ISO-8601 strings and $numberLong dates.
I'm really tempted to parse them and just return the wrong answer.  I.e. ignore leap seconds.  Windows doesn't support them (FileTimeToSystemTime always returns 0..59), plenty of operating systems don't, and even if they do it's only if they have up to date timezone files.  Doing it right involves calling tz_set before every call to mktime (so much overhead...) and all so that you can have granularity out of a string when we already support a 64 bit integer date type.

I'm not sure it's worth trying to do the right thing here