Perf: DocumentServiceLeaseManager.ListOwnedLeasesAsync -- make the query WHERE use host name

#52
mkolt
Perf warning: ListDocumentsAsync is using query anyhow, so it would make sense to change WHERE to limit Owner to this.hostName. Can be done as separate change (if choose this way, please track it using repo issue).

aery-aery
I'd prefer to do it as a separate change - this comparison was just moved from PartitionController to DocumentServiceLeaseManager, performance optimization could be done as a separate PR.