[SPARK-1385] Use existing code for JSON de/serialization of BlockId

`BlockId.scala` offers a way to reconstruct a BlockId from a string through regex matching. `util/JsonProtocol.scala` duplicates this functionality by explicitly matching on the BlockId type.
With this PR, the de/serialization of BlockIds will go through the first (older) code path.

(Most of the line changes in this PR involve changing `==` to `===` in `JsonProtocolSuite.scala`)