Remove the VdbeCursor.lastRowid cache of the current rowid, since maintaining

Remove the VdbeCursor.lastRowid cache of the current rowid, since maintaining
the correct cache value uses more CPU cycles than just recomputing the rowid
on the occasions when it is actually needed.  Replace it with the
VdbeCursor.aOffset field which used to be computed from VdbeCursor.aType
when needed. Saves 100 bytes of code space and runs 0.2% faster.

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