fix: reused instances don't keep cache

[mshanemc]

What does this PR do?

reused instances don't keep cache

explanation: RemoteSourceTrackingService(RSTS) is singleton-by-project.

queryCache is used by querySourceMembersFrom

For long-running processes (cliaas, vscode, etc) you'd potentially get wrong answers for a project by

1. getChanges (gets changes from last known revision ex: >60)
2. add/edit/delete some metadata
3. instantiate a new SourceTracking on the same project
4. getChanges (hits queryCache for 60 so it doesn't see the new changes)

with this PR, if a new instance of SourceTracking constructs instantiates a RSTS (getting it from the instanceMap) it clears the queryCache.

Result:
if you keep using the same instance of SourceTracking, the queryCache in RSTS works like current
if you instantiate a new SourceTracking, the queryCache starts empty even if it doesn't have to reconstruct a new RSTS

alternatives considered but rejected

• boolean cache param on getChanges, passed down through it
• dropping the cache entirely for querySourceMembersFrom (code comment is about the old pull command but it might apply to project retrieve 🤷

What issues does this PR fix or reference?

@W-17609556@