Merge pull request #20 from hpidcock/master

#20

This PR:\r
- Bumps to github.com/juju/mgo/v3\r
- Fixes all the unit tests (skips a few that don't work in mongo 4.4 or features we don't use i.e. MapReduce)\r
- Improve sstxn retrying on retry-able errors (added error code handling here, will update juju/txn library which has the actual retry logic)\r
- sstxn txn log writes need the same error handling as any other write op.\r
- Improve CommitTransaction and AbortTransaction by bringing across the double-tap from Mongo Driver\r
- Simplify possible Query errors returned to callers by converting net.OpError to io.EOF, this improves test stability too.\r
- Non primary write errors force a cluster sync and discard the primary socket for that session. If there is a current transaction, abort it.

Merge pull request #22 from hpidcock/v2-testing

Bring mgo juju/testing into mgo module.

Merge pull request #21 from juju/master

Merge current "v2" master into "v2"