Session: fallback failure

[coldtrick]

When for some reason there is an error in saving session data to the database it falls back to a file session.

However on the next page first Elgg tries to read from the database, as this succeeds the data in the session file can never be accessed.

To make this error happen, try to write more the 1MB of data to a session (1MB is default MySQL max_packet_size).

A possible solution would be if the session_write falls back to the file that session in the database would be removed.

[brettp]

This is probably a really rare error, so I'm pushing it to 1.9.