Remove references to Writable in the ipc package

I see references to Writable in the ipc package, most notably in the Invocation class. This class is not being used that much in the core ipc package but used in the coprocessor protocol implementations (there are some coprocessor protocols that are Writable based still). This jira is to track removing those references and the Invocation class (once HBASE-6895 is resolved).