Implement DATEDIFF

[timsehn]

From this Reddit thread. Potential customer ask:

https://www.reddit.com/r/PHP/comments/191yakg/comment/kh103cz/?context=3

https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_datediff

[fulghum]

Looks like datediff was already implemented, but our docs were out of date:

jan10/main> select dolt_version();
+----------------+
| dolt_version() |
+----------------+
| 1.31.0         |
+----------------+

jan10/main> select datediff('2007-12-31 23:59:59','2007-12-30');
+----------------------------------------------+
| datediff('2007-12-31 23:59:59','2007-12-30') |
+----------------------------------------------+
| 1                                            |
+----------------------------------------------+

I'll update our docs and fix the out of date info as part of #7277