grpc-js: Fix method config name handling in service configs

[murgatroid99]

Issue #2549 made me realize that the code that handles method config names doesn't match the specification in service_config.proto, so this fixes that. There are two related changes here:

1. The service config parser now accepts a method config name with an unset service field, and coalesces empty string to unset for both the service and method fields.
2. The code for selecting a method config by name now prioritizes the most specifically matching config, instead of the first one in the list that matches at all.