Add the OP_ReopenIdx opcode that works like OP_OpenRead except that it becomes

Add the OP_ReopenIdx opcode that works like OP_OpenRead except that it becomes
a no-op if the cursor is already open on the same index.  Update the
OR-optimization logic to make use of OP_ReopenIdx in order to avoid
unnecessary cursor open requests sent to the B-Tree layer.

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