CBG-4309: add rollback hook for cbgt to call during rollback of pIndex

[gregns1]

CBG-4309

CBGT has notion of a rollback hook that is called during rollback of pIndex at different stages. CBFT uses this in different ways but it is advised we use the hook upon rollback completion to kick the Janitor to pick up rolled back pIndexes and restart streams to the vBuckets in these pIndexes.

I believe this should fix the issue where customers rolled back pIndexes weren’t being picked up by the janitor.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2900/

[torcolvin]

Since this modifies a global in cbgt, can we move this to init where we add the other globals?

sync_gateway/base/dcp_feed_type.go

Line 119 in a68b162

func init() {

[gregns1]

I agree this makes sense but we need access to the manager to kick the janitor, which doesn't look like it can be done there?

[torcolvin]

Then we need to figure out how to run a function that passes in a manager for a specific bucket initCBGTManager is called once per bucket and then a second database would overwrite this callback for the first.

I think we might be able to do something with a wrapper function to look up mgr based on pindex and store this globally, or we could ask cbgt team to change their API so this isn't required? I don't know if they have one manager per instance, but that seems wacky.