Split WorkQueue into a separate file.

c++/Makefile.am

@@ -198,6 +198,7 @@ libkj_la_SOURCES=                                              \
 libkj_async_la_LIBADD = libkj.la $(PTHREAD_LIBS) -lpthread
 libkj_async_la_LDFLAGS = -release $(VERSION) -no-undefined
 libkj_async_la_SOURCES=                                        \
+  src/kj/work-queue.h                                          \
   src/kj/async.c++                                             \
   src/kj/async-unix.c++                                        \
   src/kj/async-io.c++

c++/src/kj/async-unix.h

@@ -83,9 +83,9 @@ protected:
   void wake() const override;
 
 private:
-  class PollItem;
+  class PollJob;
   class PollPromiseAdapter;
-  class SignalItem;
+  class SignalJob;
   class SignalPromiseAdapter;
 
   struct Impl;