Resolving imported policies in Ditto's "things-search" does not use a cache to load same policy

[thjaeckle]

During updating a single policy which is imported in ~5.000 other Policies, I noticed many "ask timeout exceptions" being thrown in Ditto's "things-search" service.

It is intended that - when a imported policy is changed - all things using policies importing from that single policy are updated in the search index (to reflect the changed permissions).

It is however quite unexpected that e.g. the same policy org.eclipse.ditto:my-root-policy is loaded for all policies which use that exact same policy. But this seems to be the case:

ditto/thingsearch/service/src/main/java/org/eclipse/ditto/thingsearch/service/persistence/write/streaming/ResolvedPolicyCacheLoader.java

Lines 49 to 50 in aba53ad

	return policy.withResolvedImports(
	importedPolicyId -> policyCacheLoader.asyncLoad(importedPolicyId, executor)

This will of course cause a huge amount of SudoRetrievePolicy commands issued at the exact same time against the policies shard region, leading to many timeouts.
This place should make use of the Policy cache, so that for each cluster instance of "things-search" a heavily "imported" policy is only loaded once.

[thjaeckle]

Resolved by: #1872