Check CapacityLimiter's handling of _pending_borrowers

[njsmith]

I was just reading the code for an unrelated reason, and realized that this code looks very suspicious:

trio/trio/_sync.py

Lines 286 to 296 in 65729b1

	try:
	self.acquire_on_behalf_of_nowait(borrower)
	except _core.WouldBlock:
	task = _core.current_task()
	self._pending_borrowers[task] = borrower
	await self._lot.park()
	except:
	await _core.cancel_shielded_checkpoint()
	raise
	else:
	await _core.cancel_shielded_checkpoint()

Specifically, if the call to await self._lot.park() gets cancelled, it looks like we don't clear self._pending_borrowers[task], which could be a memory leak (since it pins task objects in memory).