replace libev with libuv

[garlick]

After a discussion in our meeting last week, I did an experiment to convert flux-core to use libuv.

First, I did some refactoring to localize all the libev usage to reactor.c. This involved reimplementing the buffer watchers, handle watcher, and zeromq watcher using the flux API instead of libev. Then I began systematically converting reactor.c to libuv. Although, as discussed, libuv derives from liibev and thus many of its idioms carry over, I ran into the following problems:

• no equivalent to ev_child
• no equivalent to ev_periodic
• no equivalent to ev_stat (oops, I missed uv_fs_event_t)
• no equivalent to ev_set_priority()
• no equivalent to ev_ref()
• memory for handle (libuv name for watcher) objects cannot be freed in flux_watcher_destroy(). One calls uv_close() on the handle (registering a callback), the reactor runs, and someday you get a close callback, after which memory can be freed. Grr. Because Windows I guess.

It's a fair bit of work to reimplement the missing watchers though probably not a huge deal. I'm not sure what to do about the destructor issue though. It doesn't map well onto our API.

This is on my port_libuv branch if anyone wants to take a look. libflux compiles with the missing functionality. The reactor unit test runs successfully, but when run under valgrind a bunch of invalid reads show up because watcher memory is freed too soon. memory is leaked. There is also an unexplained segfault in the composite watcher test.

[vchuravy]

I hadn't had time in a while to look at flux, but from my perspective this would "fix" #3969 since the two event-loops that were fighting were libuv and libev (with some Julia idiosyncracies on top).