Merge in performance enhancements for INSERT operations, especially INSERTs

Merge in performance enhancements for INSERT operations, especially INSERTs
on tables that have no affinity columns or that have many indices or INSERTs
with content coming from a SELECT.  Add the SQLITE_TESTCTRL_VDBE_COVERAGE
test control and the SQLITE_VDBE_COVERAGE compile-time option used for measure
coverage of branches in VDBE programs.

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