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This PR introduces a new vectored implementation of the read path. The search is basically a DFS if you squint at it long enough. LayerFringe tracks the next layers to visit and acts as our stack. Vertices are tuples of (layer, keyspace, lsn range). Continuously pop the top of the stack (most recent layer) and do all the reads for one layer at once. The search maintains a fringe (`LayerFringe`) which tracks all the layers that intersect the current keyspace being searched. Continuously pop the top of the fringe (layer with highest LSN) and get all the data required from the layer in one go. Said search is done on one timeline at a time. If data is still required for some keys, then search the ancestor timeline. Apart from the high level layer traversal, vectored variants have been introduced for grabbing data from each layer type. They still suffer from read amplification issues and that will be addressed in a different PR. You might notice that in some places we duplicate the code for the existing read path. All of that code will be removed when we switch the non-vectored read path to proxy into the vectored read path. In the meantime, we'll have to contend with the extra cruft for the sake of testing and gentle releasing.