Remove the BtreeMutexArray object - use the Vdbe.btreeMask field to accomplish

Remove the BtreeMutexArray object - use the Vdbe.btreeMask field to accomplish
the same result.  Add a generation counter to btree mutexes in order to assert
that mutexes are never temporarily dropped over a range of instructions in order
to do deadlock avoidance in some subroutine.  Lock all btrees in any Vdbe
program that uses OP_ParseSchema.

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