CBG-2412: Leaking rest/replicator_test.go goroutines

[gregns1]

CBG-2412

A small PR that fixes leaking goroutines in blip sender/receiver and a http server leak. Attached is the pprof output for the replicator test package now.

• http server leak came friom the collectiosn test, missing a close afrter the creation of the object
• There was a test in the main replicator_test file that wasn't stopping a replication properly. I've changed the test to use sgr peers helper function to make use of the teardown function. Also made use of the create replication helper function as it just makes the test more readable.

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/1691/