Only wait for a single debug signal at a time as the signal state
is global. Also, do not activate the query cache debug sync points
if the thread has no associated THD session.

mysql-test/t/query_cache_debug.test:
  Only wait for a single debug signal at a time as the signal state
  is global.
sql/sql_cache.cc:
  Do not execute the debug sync point if the thread has no associated
  THD session. This scenario happens for federated threads.