use kj's onChildExit mechanism

This reduces code and allows for more idiosyncratic use of Promises.
Requires latest capnproto git.

Part of #49 refactor

test/test-run.cpp

@@ -31,7 +31,7 @@ protected:
     }
     void wait() {
         int state = -1;
-        waitpid(run.current_pid, &state, 0);
+        waitpid(run.current_pid.orDefault(0), &state, 0);
         run.reaped(state);
     }
     void runAll() {

test/test-server.cpp

@@ -66,7 +66,6 @@ public:
     MOCK_METHOD4(setParam, bool(std::string job, uint buildNum, std::string param, std::string value));
     MOCK_METHOD0(getCustomCss, std::string());
     MOCK_METHOD0(abortAll, void());
-    MOCK_METHOD0(reapChildren, void());
     MOCK_METHOD0(notifyConfigChanged, void());
 };