Copy mongo sessions before use so they don't get stale
Fix some collection names
Copy sessions in pinger
Cherry pick trunk rev a1b0ad5
Fix megawatcher inconsistency
Code review tweaks
Found another place where collection needs to copy session
Rename old statetxn imports
Merge branch '1.20' into copy-session-1.20
Fix old legacy conn tests
Fix jujutxn dependency. Precreate txns collection when connecting to mongo first up
Merge pull request #385 from wallyworld/copy-session-1.20

Copy mongo sessions

Backport from master.

Fixes lp:1307434 (hopefully)

Each time a mgo interaction is required, the session is copied to try and ensure the underlying socket is not stale.