Add the OP_FinishSeek opcode which completes an OP_DeferredSeek if the seek

Add the OP_FinishSeek opcode which completes an OP_DeferredSeek if the seek
has not already completed.  Also add the sqlite3WhereUsesDeferredSeek()
interface to the query planner.  The UPDATE implementation adds an
OP_FinishSeek before running the final OP_Insert if one is needed.
Ticket [ec8abb025e78f40c] and also an assertion fault reported by Yongheng.

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