unix, windows: rework reference counting scheme

This commit changes how the event loop determines if it needs to stay alive.

Previously, an internal counter was increased whenever a

unix, windows: rework reference counting scheme

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.

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unix: replace C99/C++ comments, fix build

Tests: verify that uv_write and uv_shutdown ref the event loop

test: add lots of refcount tests

Remove uv_init calls from tests and benchmarks

bring back uv_init

unix: multiplicity

Test that loop refs going down to zero in a prepare callback does not hang the event loop

Move loop reference tests to their own file