Support Multiple Mail Servers in Confluence

Currently it is not possible to allow multiple mail servers to be configured in Confluence. One would, for example, like to associate a specific space to a mail server or define other constraints.

According to JF in regards to configuring multiple mail servers.

• accidental feature, and we only ever actually use one of those configs (in which case, how do we know which one it uses?)
• allowing the administrator to switch between mailservers, so only one config is active at any time
• allowing a failover system in the way we use multiple LDAP servers: if the first one fails, try the next