- INVALID MARGIN CALCULATIONS
Bug#21923874 POTENTIAL RUNNING OUT OF SPACE IN REDO
             - MISSING CALLS TO MARGIN_CHECKPOINT_AGE
Bug#21923559 POTENTIAL OVERWRITE OF CHECKPOINT WITH AN OLDER CHECKPOINT

This commit does not fix these bugs. It is only adding instrumentation,
which is helpful in reproducing the listed bugs.

1. Multi-SIGNAL in DEBUG_SYNC.

It's now possible to emit multiple signals in a single execution of action:
SET DEBUG_SYNC= 'now SIGNAL a,b,c WAIT_FOR x';

Changes in documentation are required at:
https://dev.mysql.com/doc/internals/en/debug-sync-facility.html



2. Debug instrumentation for some of internal threads.

Three new global system variables have been defined in DEBUG build:
* innodb_master_thread_disabled_debug [default: FALSE],
* innodb_page_cleaner_disabled_debug [default: FALSE],
* innodb_dict_stats_disabled_debug [default: FALSE].

Reviewed-by: Marko Mäkelä <marko.makela@oracle.com>
Reviewed-by: Debarun Banerjee <debarun.banerjee@oracle.com>
RB: 10602