RFR: Eliminate string dedup cleanup phase and correct UR closure

[Zhengyu Gu]

>>
>> or we can pre-evacuate it?
> 
> The elimination of second bitmap is undecided, but we need to understand if it is absolutely
> required for String dedup to work at init mark and have complete bitmap ready, or we can untie this
> dependency.

Let me experiment pre-evacuating string dedup table.

Thanks,

-Zhengyu

> 
> -Aleksey
>