Refactor wait handles

In preparation for using wait handles in --on-process-exit events, factor
wait handles into their own wait handle store. Also switch them to
per-process instead of per-job, which is a simplification.

CMakeLists.txt

@@ -128,7 +128,8 @@ set(FISH_SRCS
     src/proc.cpp src/reader.cpp src/redirection.cpp src/sanity.cpp src/screen.cpp
     src/signal.cpp src/termsize.cpp src/timer.cpp src/tinyexpr.cpp
     src/tokenizer.cpp src/topic_monitor.cpp src/trace.cpp src/utf8.cpp src/util.cpp
-    src/wcstringutil.cpp src/wgetopt.cpp src/wildcard.cpp src/wutil.cpp src/fds.cpp
+    src/wait_handle.cpp src/wcstringutil.cpp src/wgetopt.cpp src/wildcard.cpp 
+    src/wutil.cpp src/fds.cpp
 )
 
 # Header files are just globbed.