Prune "null" support

[pmlopes]

Redis doesn't allow null in requests. Currently for compatibility reasons we encode null as the String "null". In order to make the behavior consistent with jedis the official client, we should throw an IllegalArgumentException in that case.

In order to avoid breaking changes, for now we should have a JVM flag to enforce it as strict null:

-Dio.vertx.redis.request.null-as-string=true

• deprecate all nullArg()
• if the flag is false during the request encoding if a null is present throw IAE

[pmlopes]

Initial step in commit: 05100c0