Add the "multiplex_truncate" PRAGMA to the multiplexor extension, for
querying and setting the truncate flag on a database connection.
Simplify the CellInfo structure for a size reduction and performance
improvement.
Avoid calling btreeParseCellPtr() from within fillInCell() since most of
what btreeParseCellPtr() computes is ignored by fillInCell().  Instead, have
fillInCell() compute the values it needs inline.  Performance improvement.
Remove an unused C-preprocessor macro.  No functional changes to the code.
Add the MemPage.noPayload boolean and use it to help
cellSizePtr() and btreeParseCellPtr() run faster.
Shorten all lines of source code in btree.c to at most 80 characters.
No logical changes.
Have the clearCell() routine return the cell size to the caller, rather
than have the caller make a separate call to cellSizePtr().