G1: Use more accurate integer type for collection set length

`G1CollectionSet::_collection_set_max_length` is declared `size_t` but it's initialized with a value of `uint`.

```
void G1CollectionSet::initialize(uint max_region_length) {
  ...
  _collection_set_max_length = max_region_length;
  ...
}
```

Change it to `uint` for consistency.

Additionally, `_collection_set_cur_length` can be changed to `uint` as well, because of the invariant, `_collection_set_cur_length <= _collection_set_max_length`.