Messages weren't flushed to the database when send.immediately is false

[tkvw]

I think it should honor the useFlushOnSave setting. I also added the possibility to configure it on the builder with flush(true/false).

If you want to see the effect, simply change the send.immediately to false in the example project. The to fields aren't saved to the database.

[tkvw]

Also added configurable mappings options. Your mail can now easily be stored in another datasource.

[demon101]

For asynchronous.mail.disable=true in some cases AsynchronousMailMessage not stored in DB @kefirfromperm could you include this fix to new version of plugin, please