RATIS-2287. Improve RaftServerImpl#toString() to make it more readable #86

Job	Run time
CI / build / build	1m 13s
CI / release / release	5m 9s
CI / unit (misc) / unit (misc)	6m 12s
CI / unit (grpc) / unit (grpc)	10m 0s
CI / unit (flaky) / unit (flaky)	4m 14s
CI / basic (author) / author	8s
CI / basic (rat) / rat	13s
CI / basic (findbugs) / findbugs	1m 53s
CI / basic (checkstyle) / checkstyle	21s
CI / unit (server) / unit (server)	13m 41s
CI / repro / repro	1m 14s
CI / compile (21) / compile (21)	1m 8s
CI / compile (17) / compile (17)	1m 9s
CI / compile (11) / compile (11)	1m 0s
CI / coverage	0s
	47m 35s

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