stamper index assignment rules changes

[istae]

The stamper.Stamp() should follow the rules below:

if the stamp item already exists, and the batch is mutable, reuse same index and use new timestamp.
if the stamp item already exists, and the batch is immutable, reuse the same stamp.
if the stamp does not exists, get a new index, and save the stamp item.

the other required change is that the Stamp function must be mutex protected per chunk (using multex).