IModelDb.clearCaches() should call clearECDbCache()

[soham-bentley]

Currently, IModelDb.clearCaches does not clear the native ECDb cache.

Noted by @khanaffan here

Also as per discussion with @khanaffan TxnManager._onChangesApplied should only clear instance caches, not the entire backend cache.

Fix:
IModelDb.clearCaches() should call clearECDbCache()
TxnManager._onChangesApplied should only clear instance caches
Fix tests accordingly