Fix WheelTimer implementation that can expired timeout early

[udovichenkoAlexander]

When entries insert in the end of timer queue, then unnecessary entry inserted (with duplicated key).
This can lead to some timeouts expired early and consume memory.

This can be checked with following test (to this file)

def test_multi_insert_then_past(self) -> None:
    wheel: WheelTimer[object] = WheelTimer(bucket_size=5)

    obj1 = object()
    obj2 = object()
    obj3 = object()
    wheel.insert(100, obj1, 150)
    wheel.insert(100, obj2, 160)
    wheel.insert(100, obj3, 155)
    
    self.assertListEqual(wheel.fetch(110), [])
    self.assertListEqual(wheel.fetch(158), [obj1])

Look at the entries after obj2 insert:

_Entry(end_key=21, elements=set())
_Entry(end_key=22, elements=set())
_Entry(end_key=23, elements=set())
_Entry(end_key=24, elements=set())
_Entry(end_key=25, elements=set())
_Entry(end_key=26, elements=set())
_Entry(end_key=27, elements=set())
_Entry(end_key=28, elements=set())
_Entry(end_key=29, elements=set())
_Entry(end_key=30, elements=set())
_Entry(end_key=31, elements={<object object at 0x7a129fb7ab80>})
_Entry(end_key=31, elements=set()) <-- this entry has duplicate key
_Entry(end_key=32, elements=set())
_Entry(end_key=33, elements={<object object at 0x7a129fb7ab70>})

gaps and duplicates cause errors when search Entry to insert in the middle of queue:
self.entries[max(min_key, then_key) - min_key].elements.add(obj)

This Pull request fix this with some minor refactoring (remove current_tick unused variable) and add this test case to unit tests.

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