Remove an incorrect debugging assert() that was accidently added during the
STAT4 enhancement.
Restore the hexrekey pragma which was accidently deleted during the
pragma refactoring.  Make sure the hexkey and hexrekey pragmas do not
overflow buffers with a over-length key.
Merge bug fixes from trunk.
Multiply all cursor step cost estimates by the estimated size of the row in
bytes, in order to get the query planner ot make use of estimated row sizes.
This check-in uses magic numbers in a few places (for example, estimates of
the size of output rows) and needs lots of refinement.  Consider this a
proof-of-concept only.