The Patch() method was intended to skip any records that do not
currently exist. However, currently inmemory.Backend.Patch() will return
ErrNotFound if the last record in the records slice is not found (it
will ignore any other previous records that are not found).
Update the error handling logic here to be consistent with the postgres
backend, and add a unit test to exercise this case.
Add a Patch() method to the databroker gRPC service.
Update the storage.Backend interface to include the Patch() method now
that all the storage.Backend implementations include it.
Add a test to exercise the patch method under concurrent usage.
Add a new Patch() method that updates specific fields of an existing
record's data, based on a field mask.
Extract some logic from the existing Get() and Put() methods so it can
be shared with the new Patch() method.
