Scope: move the truncate-related fields out of the layered table (WT_LAYERED_TABLE) into a separate structure.

Why we do this — a step toward giving the layered-cursor per-operation context (WT_CLAYERED_OP) and its internal functions only the state they actually need, not the whole table or cursor:

• Don't hand functions more information than they use — the op can then carry the truncate list, not the full WT_LAYERED_TABLE.
• Simplify unit testing of layered cursors: a self-contained WT_CLAYERED_OP can be exercised directly, without mocking the entire layered table.