Deprecate mysql.NullTime

[julienschmidt]

database/sql will have a NullTime type from Go1.13 onward, which should be used instead: golang/go#30305

[dolmen]

Or better: make mysql.NullTime an alias to database/sql.NullTime when compiling on Go 1.13+.

I will submit a PR to implement this solution.

[dolmen]

Well, the Scan method is specific to MySQL, so we have to keep it. I will change to wrap database/sql.NullTime.

[AlaNDGT]

Will rows.ColumnTypes() (first value from db.Query()) return sql.NullTime instead of mysql.NullTime too?