Various DB access improvements to BounceHandler extension

* Added $wgBounceHandlerSharedDB to support sharing the bounce table
* Made exim to talk back only to meta wiki
* Removed SMTP settings as its not used in production

Change-Id: Icc40ac29f6b0171d3736589d538b0dfdcede3706
Merge "Various DB access improvements to BounceHandler extension"