Updates netty version to 4.1.115.Final #757

Job	Run time
Full verify (8)	2m 11s
Setup ITs	3s
Unit tests (8)	2m 33s
Build (8)	2m 8s
Cassandra ITs (RELEASE-3.X, 8)	21m 35s
Scylla ITs (ENTERPRISE-RELEASE, 8)	10m 45s
Cassandra ITs (RELEASE-4.X, 8)	27m 17s
Scylla ITs (ENTERPRISE-PRIOR-RELEASE, 8)	30m 6s
Scylla ITs (OSS-RELEASE, 8)	10m 25s
Scylla ITs (OSS-PRIOR-RELEASE, 8)	10m 46s
	1h 57m 49s

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