Fixed MDEV-8428: Mangled DML statements on 2nd level slave when enabling binlog checksums

Fix was to add a test in Query_log_event::Query_log_event() if we are using
CREATE ... SELECT and in this case use trans cache, like we do on the master.
This avoid using (with doesn't have checksum)

Other things:
- Removed dummy call my_checksum(0L, NULL, 0)
- More DBUG_PRINT
- Cleaned up Log_event::need_checksum() to make it more readable (similar as in MySQL 5.6)
- Renamed variable that was hiding another one in create_table_imp()

sql/rpl_utility.cc

@@ -1174,7 +1174,7 @@ bool event_checksum_test(uchar *event_buf, ulong event_len, uint8 alg)
       compile_time_assert(BINLOG_CHECKSUM_ALG_ENUM_END <= 0x80);
     }
     incoming= uint4korr(event_buf + event_len - BINLOG_CHECKSUM_LEN);
-    computed= my_checksum(0L, NULL, 0);
+    computed= 0;
     /* checksum the event content but the checksum part itself */
     computed= my_checksum(computed, (const uchar*) event_buf, 
                           event_len - BINLOG_CHECKSUM_LEN);