multiprocess: Add echoipc RPC method and test

Add simple interfaces::Echo IPC interface with one method that just takes and
returns a string, to test multiprocess framework and provide an example of how
it can be used to spawn and call between processes.

test/functional/rpc_misc.py

@@ -61,6 +61,9 @@ class RpcMiscTest(BitcoinTestFramework):
         node.logging(include=['qt'])
         assert_equal(node.logging()['qt'], True)
 
+        self.log.info("test echoipc (testing spawned process in multiprocess build)")
+        assert_equal(node.echoipc("hello"), "hello")
+
         self.log.info("test getindexinfo")
         # Without any indices running the RPC returns an empty object
         assert_equal(node.getindexinfo(), {})