mysql: add syncer.EventCacheCount setting #19638

Job	Run time
flow_test (ubuntu-latest-16-cores, 18, maria, 8.0, latest)	15m 44s
flow_test (ubuntu-latest-16-cores, 17, mysql-pos, 7.0, stable)	18m 33s
flow_test (ubuntu-latest-16-cores, 16, mysql-gtid, 6.0, lts)	15m 59s
	50m 16s

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