Harmonize request and response types

Add a 'design decision' (rule) that we use FooBarRequest and FooBarResponse for rpc FooBar.

Reasons:

• it avoids coupling between RPC's
• it avoids discussion while doing parallel work on different RPC's

This pattern does not seem to be uncommon.

• https://github.com/apache/mesos/blob/ea3436b38c59d4211abde589adc8e3284a5cca20/include/mesos/mesos.proto
• https://github.com/coreos/etcd/blob/43740a8d3c36f37f42754074856b7fc1c76beab5/etcdserver/etcdserverpb/rpc.proto