avoid session flush problems in AsynchronousMailService

avoid session flush problems in AsynchronousMailService adding 'withNewSession' to the 'Save message to DB' section
asynchronous.mail.newSessionOnImmediateSend config

asynchronous.mail.newSessionOnImmediateSend config added to AsynchronousMailService
asynchronous.mail.newSessionOnImmediateSend default config

Added asynchronous.mail.newSessionOnImmediateSend in default config
Merge pull request #32 from aberbenni/master

Fix GPASYNCHRONOUSMAIL-33