Dynamic tables, on concrete need: versioned reads, ordered-table specifics, a migrate helper

[sshaplygin]

Part of #63.

A placeholder, deliberately unscheduled. Filed so the dynamic-table list is not silently short, and closed unbuilt if nothing ever asks for these. None of it starts before something concrete needs it.

• versioned and timestamped reads — the timestamp is already carried on the wire; what is missing is the versioned rowset decode, which docs/rpc-compatibility.md marks not implemented, only on concrete need
• ordered-table specifics: tablet_index on writes, and trim_rows (C++ has TrimRows; Go does not)
• a migrate-style schema-evolution helper of the kind the Go SDK ships — mount state, schema diff, and the unmount/alter/mount dance done once and correctly