Changing default failure behavior

[anthony]

I just pushed a change to make the 'must_succeed' option default to
'yes'. If you do not want this behavior, you can override it by
setting 'must_succeed=no' in each top-level store.

Previously, (if you did not specify this parameter), if a store
failed, all incoming messages would be dropped. We recommend always
using a buffer store so that scribe would have a backup location to
store messages. However, it is still possible that both the primary
store and secondary store will fail and messages would be dropped.
But by enabling must_succeed, Scribe will never drop messages and will
instead tell the upstream client to try again later.

I think enabling this behavior by default is the right decision for
most users. The only reason you would want to override this behavior
is if you had a store where you would rather have it drop messages on
failure instead of having scribe stop accepting new messages.

-Anthony