sql: introduce DATETIME to SQL

This patch introduces basic DATETIME support in SQL. After this patch,
it will be allowed to select DATETIME values from spaces, insert them
into spaces, and use them in functions. CAST() from DATETIME to STRING,
SCALAR and ANY is also supported.

Part of #6773

NO_DOC=Doc-request will be added in another commit.
NO_CHANGELOG=Changelog will be added in another commit.