Fix connection issues with CONN_MAX_AGE > 0

[msabatier]

For proper operation with CONN_MAX_AGE is > 0 we have to make sure that connection objects are no shared between parent and child processes.

Proposed change is to replace db.connection.close() by db.connections.close_all() in spawn_cluster().

Related open issues : #589 #486 #484 #471 #435

This has only been tested on linux, not on MacOS or Windows, as I currently don't have a way to test on those environments.

[aaronn]

Would be great to get this one merged.