This commit changes how the event loop determines if it needs to stay alive.
Previously, an internal counter was increased whenever a handle got created
and decreased again when the handle was closed.
While conceptually simple, it turned out hard to work with: you often want
to keep the event loop alive only if the handle is actually doing something.
Stopped or inactive handles were a frequent source of hanging event loops.
That's why this commit changes the reference counting scheme to a model where
a handle only references the event loop when it's active. 'Active' means
different things for different handle types, e.g.:
* timers: ticking
* sockets: reading, writing or listening
* processes: always active (for now, subject to change)
* idle, check, prepare: only active when started
This commit also changes how the uv_ref() and uv_unref() functions work: they
now operate on the level of individual handles, not the whole event loop.
The Windows implementation was done by Bert Belder.
This patch changes how uv-win uses the UV_SHUTTING and UV_SHUT flags.
UV_SHUT is now only used for tcp handles to track whether shutdown() has
actually been called. UV_SHUTTING has the more generic meaning of
"no longer readable". It would be good to replace it by an actual
UV_READABLE flag in the future.
This makes the shutdown_close_tcp and shutdown_close_pipe tests pass on
windows.
