Enhance upgrade tests to include mid-upgrade restart

[adamcfraser]

In SG 2.0 we have support for restarting DCP processing midway through a large backfill. This is particularly relevant for these cases:

• Initial import of a large bucket of non-mobile data
• Initial migration of a large bucket from docmeta to xattrs

These are currently covered by upgrade tests (I believe). We should enhance those tests to validate the restart capabilities, by restarting the SG node midway through import/migration. Test should validate that when restarted, SG isn't restarting the import/migration from zero. There are new Sync Gateway expvars available to monitor DCP backfill progress - these could be used to evaluate this test.

[tleyden]

Related to couchbase/sync_gateway#3184