If we're going to hav valgrind verify state streams then we need to ensure
that once we choose a pointer into a block we always use that pointer until
the block is freed. I was trying to do this with the "current_map" thing.
However, that breaks down because you have to use the map from the block
pool to get to the stream_block to get at current_map. Instead, this
commit changes things to track the stream_block by pointer instead of by
offset into the block pool.
e6fc170afb