Performance tip to disable tracking

[habbes]

Every so often a customer using OData Client in a long-running service reports consistent high memory usage after making a request that returns a large payload. This issue can usually be resolved by disabling entity tracking and/or using short-lived/scoped DataServiceContext instances (e.g. one per request) instead of a long-lived shared one. We should add a section in the perfomance guidelines doc that talks about this.

Sample customer issue: OData/odata.net#3019