MDEV-31273: Eliminate Log_event::checksum_alg

This is a preparatory commit for pre-computing checksums outside of
holding LOCK_log, no functional changes.

Which checksum algorithm is used (if any) when writing an event does not
belong in the event, it is a property of the log being written to.

Instead decide the checksum algorithm when constructing the
Log_event_writer object, and store it there.

Introduce a client-only Log_event::read_checksum_alg to be able to
print the checksum read, and a
Format_description_log_event::source_checksum_alg which is the
checksum algorithm (if any) to use when reading events from a log.

Also eliminate some redundant `enum` keywords on the enum_binlog_checksum_alg
type.

Reviewed-by: Monty <monty@mariadb.org>
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

sql/rpl_utility.cc

@@ -296,7 +296,7 @@ table_def::~table_def()
 
 */
 bool event_checksum_test(uchar *event_buf, ulong event_len,
-                         enum enum_binlog_checksum_alg alg)
+                         enum_binlog_checksum_alg alg)
 {
   bool res= FALSE;
   uint16 flags= 0; // to store in FD's buffer flags orig value