[FTS] QueryString syntax doesn't have short way to perform exact date equality search

This issue is like [mb-22416], except that was numbers in general and this is for dates specifically.

Today, a query string query to look for an exact date value needs to use a bounded approach, using both '<=' and '>=' numeric range operators. The fix for mb-22416 lets you do an exact numeric match, but that doesn't completely work for dates.

The additional issue with dates is that there needs to be some way to indicate the degree of precision, otherwise the dates would need to be the same down the millisecond.

For example, imagine an FTS index on beer-sample that indexes brewery objects. Brewery has an field called "updated".

Possible values might be:

"updated": "2010-10-24"
"updated": "2010-10-24 13:54:07"

 A user might expect a search like this to match both entries:
+updated=2010-10-24