Release memory where possible in RestElasticSearchClient #5366

Job	Run time
build-all	3m 10s
build-java11	2m 54s
tests (core, 8)	1m 37s
tests (driver, 8)	1m 11s
tests (server, 8)	3m 49s
tests (test, 8)	6m 43s
tests (inmemory, -Dtest.skip.tp=false, 8)	32m 1s
tests (berkeleyje, 8)	9m 7s
tests (lucene, 8)	4m 55s
tests (core, -Pjava-11, 11)	1m 30s
tests (driver, -Pjava-11, 11)	1m 13s
tests (server, -Pjava-11, 11)	3m 54s
tests (test, -Pjava-11, 11)	7m 7s
tests (inmemory, -Pjava-11, -Dtest.skip.tp=false, 11)	32m 17s
tests (berkeleyje, -Pjava-11, 11)	9m 10s
tests (lucene, -Pjava-11, 11)	5m 0s
	2h 5m 38s

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