Internal kvvfs docs and cleanups. Wasm speedtest1: remove the hard-coded --size=2...

Internal kvvfs docs and cleanups. Wasm speedtest1: remove the hard-coded --size=2 limit for kvvfs and use a transient storage object instead of session storage (which is limited to --size=2). For small job sizes kvvfs is roughly 3x the speed of memdb and that gap grows as the --size increases, with kvvfs _really_ bogging down on the --size=50 jsonb tests.

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