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'.