Simplify date/datetime datatypes

[sundy-li]

Proposal:

1. Union Date(Date16) Date32 to be Date type, which uses i32 as physical datatype.
2. Union DateTime (DateTime32), DateTime64 to be DateTime type, which uses i64 as physical datatype.

[BohuTANG]

For 2., in duckdb is timestamp:
https://duckdb.org/docs/sql/data_types/timestamp

In snowflake:

DATETIME is an alias for TIMESTAMP_NTZ.