Revert "Fixed #18135 -- Close connection used for db version checking"

This reverts commit 4423757c0c50afbe2470434778c8d5e5b4a70925.
Fixed #15119 -- Stopped pinging the MySQL server.
Removed duplicate caching of mysql_version.

The manual caching in self.server_version and the cached_property
decorator are redundant.
Implemented persistent database connections.