Use Python allocators for the event loops

libuv 1.34.0

Even before this it was often right at the border on Py38 for some reason.

Fixes #1506.

We don't use the uv_random interfaces so exclude those files.

we don't support haiku so exclude that file.

The binary mode patch for #1282 is no longer needed after https://github.com/libuv/libuv/pull/2324

Not a 100% fix, but takes care of this.

Seems to have some sort of regression on libuv/windows, but only in
test__backdoor, which is weird anyway. I can't reproduce locally, and
test__backdoor is often flaky on windows anyway, so we'll skip it for now.

Not clear why.

Fixes #1504

Update tested Python versions

[skip ci]

And stop a fresh greenlet from even running.

Fixes #1473.

Free user-provided resources as soon as the greenlet is cancelled, if it's not running.

Fix potential crashes in the FFI backends if a watcher was closed and…

Fix potential crashes in the FFI backends if a watcher was closed and stopped in the middle of a callback from the event loop and then raised an exception.

This could happen if the hub's ``handle_error`` function was poorly customized, for example.

Fixes #1482

Direct error handling through the loop in the unexpected-but-closed case as well.

Fix threadpool in command line scripts

Keep the actual greenlet objects, not just a counter. This way we can clean up after them better.

Only happens on Travis. Recently, the failure has started including 'Fatal Python error: PyImport_GetModuleDict missing'.

Fixes #1484.

In general, fixes any use of gevent.monkey.get_original() in such a process.

Be more careful about double-patching in general too. Detecting duplicate arguments to patch_all only patches things that need to be patched.

Fix an interaction between the switch interval and libuv that could introduce delays for large batches of callbacks

Fix an interaction between the switch interval and libuv that could introduce delays processing large batches of callbacks.

Specifically, a .3 (idle signal timer) delay as UV_RUN_ONCE would pause if there was no other active timer or IO watcher.

Now, if there are still batches of callbacks to run, we explicitly use UV_RUN_NOWAIT to only poll for IO and queue callbacks without waiting at all.

This gets the time for the synthetic greenlet-launching benchmarks to match libev-cffi and be close to libuv-cext.

Fixes #1493