When we have label sets that are not in the correct order, fixing that
changes the order of the series in the index.  So the index must be
rewritten in that new order.  This makes this repair tool take up a
bunch more memory, but produces blocks that verify correctly.

The directory name must be the block ID name exactly to verify.  A temp
directory or random name will not work here.

Pointer/reference logic error was eliminating all chunks for a series in
a given TSDB block that wasn't the first chunk.  Chunks are now
referenced correctly via pointers.

Some linters catch errors.Errorf() as its not really part of the errors
package.

We're comparing items by pointers, using Go's range variables is
misleading here and we need not fall into the same trap.

This prevents us from having to re-implement label sorting.