Performance

Use work item field instead of store for work item type. The new IWorkItem interface has a property named WorkItemType, which contains the name of the work item type for that work item. This is equivalent to the property IWorkItem.Type.Name, except that the work item store does not receive a query to load WIT meta data for just the name.
Mapper does not require work item type field definitions to build cache. When building the property cache for a work item type and model, the attribute mapper would verify the decorated property/field existed as a field in the WIT. This is not necessary, as an invalid field would return null. In REST, we use the object accessor to get the initial field value, which would try to load the WIT and fail. WorkItemCore has been updated to catch the InvalidOperationException thrown by the REST client and attempt to read values from that point forward using the dictionary supplied by the REST API.

Bug fixes

Better error messaging. Provide more descriptive error messages when certain preconditions are not met during query (e.g. a field is missing)