WL#4259 - Debug Sync Facility

Backport from 6.0 to 5.1.
Only those sync points are included, which are used in debug_sync.test.

  The Debug Sync Facility allows to place synchronization points
  in the code:
  
  open_tables(...)
  
  DEBUG_SYNC(thd, "after_open_tables");
  
  lock_tables(...)
  
  When activated, a sync point can
  
  - Send a signal and/or
  - Wait for a signal
  
  Nomenclature:
  
  - signal:            A value of a global variable that persists
                       until overwritten by a new signal. The global
                       variable can also be seen as a "signal post"
                       or "flag mast". Then the signal is what is
                       attached to the "signal post" or "flag mast".
  
  - send a signal:     Assign the value (the signal) to the global
                       variable ("set a flag") and broadcast a
                       global condition to wake those waiting for
                       a signal.
  
  - wait for a signal: Loop over waiting for the global condition until
                       the global value matches the wait-for signal.
  
  Please find more information in the top comment in debug_sync.cc
  or in the worklog entry.

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