This commit is a port from v2's 9a515b9234 ([SCHEMA] Improve timestamp storage) to v3.
As explained by Alexei Sorokin:
Avoid the use of deprecated MariaDATABASE "zero dates" globally. If they're present
as attribute defaults somewhere, they will be replaced with NULL implicitly.
The existing "zero dates" in MariaDATABASE storage will be left intact and this
should not present any issues.
The "timestamp" type in table definitions now corresponds to DATETIME in
MariaDATABASE with "DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP", which
should be close enough to the original behaviour for compatibility purposes.
It is now the recommended type for "modified" attributes, because of the
update trigger on MariaDATABASE. But there is no such trigger implemented on
PostgreSQL as of this moment.
This has the benefit of requiring fewer code changes, as well as providing a better isolation
between GNU social and symfony, useful in case the framework needs to be changed
