Ensure that the correct number of columns in a UNIQUE index are checked for
uniqueness, regardless of whether or not the original table has a ROWID or
if the columns are NOT NULL, etc.  Ticket [9a6daf340df99ba93c].
Add the "eForce" parameter to the sqlite3_multiplex_shutdown() entry point
in test_multiplex.c.  Shutdown is forced if true.  Shutdown is not done if
there are pending database connections and eForce is false, but an error log
entry is made instead.
Add three new static mutexes for use by the application.  This is a partial
import of changes from the threads branch.
Re-integrate the recent changes from the 'winMutex' branch back into the Win32 mutex subsystem.