allowing application to set transaction as PA unsafe

Resetting pa_unsafe flag after fragment certification. If the flag is not reset,
it would remain in effect until the end of the transaction. However, the PA safety
should be inspected only during fragment applying time.

src/transaction.cpp

@@ -1618,6 +1618,7 @@ int wsrep::transaction::certify_fragment(
         assert(state_ == s_certifying);
         state(lock, s_executing);
         flags(flags() & ~wsrep::provider::flag::start_transaction);
+        flags(flags() & ~wsrep::provider::flag::pa_unsafe);
     }
     return ret;
 }