RATIS-2240. updatePurgeIndex doesn't need to hold RaftLogBase.writeLo… #3758

Job	Run time
CI / build / build	1m 13s
CI / basic (author) / author	12s
CI / basic (findbugs) / findbugs	1m 45s
CI / basic (checkstyle) / checkstyle	23s
CI / basic (rat) / rat	15s
CI / unit (misc) / unit (misc)	7m 20s
CI / unit (grpc) / unit (grpc)	11m 45s
CI / unit (server) / unit (server)	14m 18s
CI / repro / repro	1m 6s
CI / compile (11) / compile (11)	1m 6s
CI / compile (21) / compile (21)	1m 4s
CI / compile (17) / compile (17)	1m 8s
CI / coverage	0s
	41m 35s

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