Processing set name formatting

[tnakazato]

This commit aligns number of digit for ms_v4_id. If number of partitions is 11, for example, name of the msv4 will change from "ps_name_0" to "ps_name_00", etc. That affects sort order of the msv4 on disk. Previously, it was like the following.

ps_name_0
ps_name_1
ps_name_10
ps_name_2
...
ps_name_9

After this change, the order will be more intuitive.

ps_name_00
ps_name_01
ps_name_02
...
ps_name_09
ps_name_10

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