Description of problem:
Satellite CV publish and promote waits hours for a pulp tasks unassociate_by_criteria for docker repos, in case there are >1M docker tag units stored.

The cause is, the unassociate_by_criteria task queries or deletes from units_docker_tag mongo collection units based on filter/query with repo_id and manifest_digest, like:

Oct 18 07:20:38 satellite007 mongod.27017[2985]: [conn80] command pulp_database.$cmd command: delete { delete: "units_docker_tag", ordered: true, deletes: [ { q: { repo_id: "1-mycv-library-f2183c89-ff33-4d1f-a80f-545aecf105e9", manifest_digest: "sha256:ab3a77ab3a77ab3a77ab3a77ab3aa0604477deab3a773d7f81a060440cfb81d3" }, limit: 0 } ] } numYields:0 reslen:44 locks:{ Global: { acquireCount: { r: 11753, w: 11753 } }, Database: { acquireCount: { w: 11753 }, acquireWaitCount: { w: 8 }, timeAcquiringMicros: { w: 901 } }, Collection: { acquireCount: { w: 11753 } } } protocol:op_query 2150ms

But the mongo has just the default index on this collection:

> db.units_docker_tag.getIndices()
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "pulp_database.units_docker_tag"
	},
	{
		"v" : 1,
		"unique" : true,
		"key" : {
			"name" : 1,
			"repo_id" : 1,
			"schema_version" : 1,
			"manifest_type" : 1
		},
		"name" : "name_1_repo_id_1_schema_version_1_manifest_type_1",
		"ns" : "pulp_database.units_docker_tag",
		"background" : false
	}
]
> 


Therefore, to improve performance of those pulp tasks AND consequently of CV publish/promote (that has bigger docker repos), I suggest adding relevant index.


Version-Release number of selected component (if applicable):
Sat6.5 (applicable to 6.6 as well)


How reproducible:
100%


Steps to Reproduce:
1. Populate your Satellite with big docker repo(s) with hundreds of thousands of docker tags - such that units_docker_tag collection has 1M-ish records
2. Add a docker repo to a CV and publish/promote the repo
3. Wait..


Actual results:
CV publish/promote is expected to take a long time; meanwhile /var/log/messages will have mongo long queries logged like the above one


Expected results:
CV publish/promote in a reasonable time


Additional info:
(please ensure the index is present in mongo for pulp2 and in postgres in pulp3, to prevent future performance regression)