Use mockito instead of powermock #1423

Job	Run time
PR Validation	8m 18s
Build with macos on JDK 11	5m 32s
Integration Tests	20m 45s
Bookie Tests	18m 43s
Client Tests	34m 45s
Backward compatibility tests	27m 29s
Build with windows on JDK 11	6m 11s
Replication Tests	22m 44s
Compatibility Check Java8	20m 30s
Remaining Tests	18m 16s
Compatibility Check Java11	20m 17s
TLS Tests	13m 18s
Compatibility Check Java17	20m 19s
OWASP Dependency Check	0s
StreamStorage Tests	19m 17s
Shell tests	32s
	4h 16m 56s

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