Option to clean up log files in vacuumIndex

[sezruby]

Currently, an index was removed & vacuumed, its log files are not removed after that.
So if a new index with the same index name is created, the log number is continued.
It was intended to keep the log files around (compliance reason)(#389 (comment))

However, this might cause some confusion when the new index is completely different from the previous index (index type, source dataset, ..)
For example, when collecting available index versions (#389), irrelevant previous versions can be collected.
Also it's possible that getLatestStableLog refers the previous log if all logs of the current index are not stable.

Therefore, it would be better to provide an option to remove all log files when vacuumIndex.

[imback82]

Whether we vacuum log files or not, we should update getLatestStableLog to handle the last vacuum operation correctly?

[sezruby]

Whether we vacuum log files or not, we should update getLatestStableLog to handle the last vacuum operation correctly?

Created a quick fix #398