DRILL-8515: Upgrade to HBase client 2.6.1 #4119

Job	Run time
Run checkstyle and generate protobufs	19m 9s
Run checkstyle and generate protobufs	19m 9s
Main Build (8, default-hadoop)	1h 20m 2s
Main Build (8, default-hadoop)	1h 23m 46s
Main Build (11, default-hadoop)	1h 18m 29s
Main Build (11, default-hadoop)	1h 18m 29s
Main Build (17, default-hadoop)	1h 19m 5s
Main Build (17, default-hadoop)	1h 19m 5s
Main Build (8, hadoop-2)	1h 26m 51s
Main Build (8, hadoop-2)	1h 26m 51s
	11h 30m 56s

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