Dump bridges to file, despite lack of BridgeHistory

We only dumped the bridges assigned to buckets if we had
bridgehistory for them. This is not so useful if we disable
the stability code which tracks this. We now dump all bridges
that are assigned to a distributor to file when the stability
code is disabled.
Add a test for Buckets

The others will require mocking a DB connection.
Merge remote-tracking branch 'tpo-sysrqb/bug12781-dumpbuckets' into develop