Streams: delta encode IDs based on key. Add count + deleted fields.
We used to have the master ID stored at the start of the listpack, however using the key directly makes more sense in order to create a space efficient representation: anyway the key at the radix tree is very unlikely to change because of how the stream is implemented. Moreover on nodes merging, to rewrite the merged listpacks is anyway the most sensible operation, and we can use the iterator and the append-to-stream function in order to avoid re-implementing the code needed for merging. This commit also adds two items at the start of the listpack: the number of valid items inside the listpack, and the number of items marked as deleted. This means that there is no need to scan a listpack in order to understand if it's a good candidate for garbage collection, if the ration between valid/deleted items triggers the GC.
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