Allow a functionality to disable POP Mail globally

A customer found that when running Confluence behind a firewall, a POP mail request from a space (which won't work because of the firewall) will cause the java process for Confluence to max out, bringing their whole system down.

As it is not possible to check which space that has POP configured, trying to fix a problem caused by such configuration can be very difficult.

In such a situation, it is desirable to have a way to identify whether a POP account has been setup in any spaces and also a way to disable them globally.

A work around that is available at the moment is to disable mail pooling in Confluence - this will prevent Confluence from checking for new mail automatically, but mail retrieval will still work if a user triggers "Check for new mail" manually.

This can be done by restarting Confluence with the following parameter:

-Dconfluence.disable.mailpolling=true