Create a nodetool upgrade_sstables to avoid using scrubs for tasks it wasn't intended to.

Scrub was intended to check a data file is not corrupted and to try to correct some form of corruption and discard the data when it can't repair. But we are now using it also for:

• major upgrade, to have sstable in the new data format for streaming sake (that one could be "fixed" independently by supporting old format during streaming)
• to force the compaction of existing sstables after changing the compression algorithm

We should probably provide a separate tool/command for those two last tasks since:

• we could have a better name, like upgrade_sstables or rewrite_sstables for that operation
• we could avoid the automatic snapshot that scrub does (and is not expected by users for those operations)
• make it slightly quicker/simpler by avoiding the corruption detection code