RATIS-2403. Support leader batch write to improve linearizable follower read throughput #673

Job	Run time
CI / release / release	5m 33s
CI / build / build	1m 35s
CI / basic (author) / author	13s
CI / basic (rat) / rat	20s
CI / basic (findbugs) / findbugs	2m 22s
CI / unit (flaky) / unit (flaky)	3m 39s
CI / basic (checkstyle) / checkstyle	29s
CI / unit (server) / unit (server)	12m 20s
CI / unit (grpc) / unit (grpc)	12m 18s
CI / unit (misc) / unit (misc)	11m 47s
CI / repro / repro	1m 40s
CI / compile (11) / compile (11)	1m 2s
CI / compile (21) / compile (21)	1m 3s
CI / compile (17) / compile (17)	1m 3s
CI / compile (8) / compile (8)	50s
CI / coverage	1s
	56m 15s

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