provide hidden way to trigger a complete backfill

Now that XDCR has a backfill mechanism, provide a way to manually trigger a backfill and print out the mappings for debugging if data were ever to be missing

To trigger an "emergency" backfill, edit the replication setting and pass "manualBackfill" key with the source namespace.

For example, given an explicit mapping set up of:

{

  "checkpointInterval": 60,

  "colMappingRules": {

    "S1.col1": "S1.col1"

  },

  "collectionsExplicitMapping": true,

  "collectionsMigrationMode": false,

  "collectionsMirroringMode": false,

}

Trigger a backfill by setting Key value of "manualBackfill=S1.col1"
The key is "manualBackfill" and value is the source namespace.