RATIS-2245. Ratis should wait for all apply transaction futures before taking snapshot and group remove #6

Job	Run time
CI / build / build	1m 24s
CI / unit (grpc) / unit (grpc)	11m 35s
CI / unit (misc) / unit (misc)	7m 22s
CI / unit (server) / unit (server)	13m 45s
CI / basic (author) / author	7s
CI / basic (checkstyle) / checkstyle	24s
CI / basic (findbugs) / findbugs	1m 54s
CI / basic (rat) / rat	16s
CI / repro / repro	1m 7s
CI / compile (21) / compile (21)	1m 5s
CI / compile (11) / compile (11)	1m 2s
CI / compile (17) / compile (17)	1m 3s
CI / coverage	0s
	41m 4s

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