Cleanup: make slave_exec_mode of its enum type and pack Log_event better

Pack these fields together:
event_owns_temp_buf
cache_type
slave_exec_mode
checksum_alg

Make them bitfields to fit a single 2-byte hole.

This saves 24 bytes per event.

SLAVE_EXEC_MODE_LAST_BIT is rewritten as

> SLAVE_EXEC_MODE_LAST= SLAVE_EXEC_MODE_IDEMPOTENT

to avoid a false-positive -Wbitfield-enum-conversion warning:
Bit-field 'slave_exec_mode' is not wide enough to store all enumerators of
'enum_slave_exec_mode'.

sql/log_event.h

@@ -54,6 +54,8 @@
 #include "rpl_record.h"
 #include "rpl_reporting.h"
 #include "sql_class.h"                          /* THD */
+#else
+typedef ulong enum_slave_exec_mode;
 #endif
 
 #include "rpl_gtid.h"
@@ -1249,12 +1251,6 @@ public:
      event's type, and its content is distributed in the event-specific fields.
   */
   uchar *temp_buf;
-  
-  /*
-    TRUE <=> this event 'owns' temp_buf and should call my_free() when done
-    with it
-  */
-  bool event_owns_temp_buf;
 
   /*
     Timestamp on the master(for debugging and replication of
@@ -1284,13 +1280,29 @@ public:
   */
   uint16 flags;
 
+  /**
+    true <=> this event 'owns' temp_buf and should call my_free() when done
+    with it
+  */
+  bool event_owns_temp_buf;
+
   enum_event_cache_type cache_type;
 
   /**
     A storage to cache the global system variable's value.
     Handling of a separate event will be governed its member.
   */
-  ulong slave_exec_mode;
+  enum_slave_exec_mode slave_exec_mode;
+
+  /**
+     The value is set by caller of FD constructor and
+     Log_event::write_header() for the rest.
+     In the FD case it's propagated into the last byte
+     of post_header_len[] at FD::write().
+     On the slave side the value is assigned from post_header_len[last]
+     of the last seen FD event.
+  */
+  enum enum_binlog_checksum_alg checksum_alg;
 
   Log_event_writer *writer;
 
@@ -1436,16 +1448,6 @@ public:
     return read_log_event(file, packet, fdle, checksum_alg, get_max_packet());
   }
 
-  /* 
-     The value is set by caller of FD constructor and
-     Log_event::write_header() for the rest.
-     In the FD case it's propagated into the last byte 
-     of post_header_len[] at FD::write().
-     On the slave side the value is assigned from post_header_len[last] 
-     of the last seen FD event.
-  */
-  enum enum_binlog_checksum_alg checksum_alg;
-
   static void *operator new(size_t size)
   {
     extern PSI_memory_key key_memory_log_event;

sql/log_event_server.cc

@@ -544,8 +544,8 @@ int append_query_string(CHARSET_INFO *csinfo, String *to,
 **************************************************************************/
 
 Log_event::Log_event(THD* thd_arg, uint16 flags_arg, bool using_trans)
-  :log_pos(0), temp_buf(0), exec_time(0), thd(thd_arg),
+  :log_pos(0), temp_buf(0), exec_time(0),
-   checksum_alg(BINLOG_CHECKSUM_ALG_UNDEF)
+   checksum_alg(BINLOG_CHECKSUM_ALG_UNDEF), thd(thd_arg)
 {
   server_id=	thd->variables.server_id;
   when=         thd->start_time;
@@ -569,7 +569,7 @@ Log_event::Log_event(THD* thd_arg, uint16 flags_arg, bool using_trans)
 
 Log_event::Log_event()
   :temp_buf(0), exec_time(0), flags(0), cache_type(EVENT_INVALID_CACHE),
-   thd(0), checksum_alg(BINLOG_CHECKSUM_ALG_UNDEF)
+   checksum_alg(BINLOG_CHECKSUM_ALG_UNDEF), thd(0)
 {
   server_id=	global_system_variables.server_id;
   /*
@@ -5099,7 +5099,7 @@ int Rows_log_event::do_apply_event(rpl_group_info *rgi)
       bitmap_set_bit(table->write_set, table->s->vers.end_fieldno);
     }
 
-    this->slave_exec_mode= slave_exec_mode_options; // fix the mode
+    this->slave_exec_mode= (enum_slave_exec_mode)slave_exec_mode_options;
 
     // Do event specific preparations 
     error= do_before_row_operations(rgi);