Do not reinitialize `Hive` when opening a `BoxCollection` #202

Job	Run time
ensure-embedme	5s
ensure-codegen	2m 41s
check-score (hive)	1m 33s
code-quality (hive, stable)	1m 13s
check-score (hive_flutter)	2m 9s
code-quality (hive, beta)	1m 10s
test-hive (vm, kernel)	54s
check-score (hive_generator)	2m 41s
code-quality (hive_flutter, stable)	1m 31s
test-hive (chrome, dart2js)	2m 41s
test-hive_flutter (stable)	50s
test-hive (chrome, dart2wasm)	2m 41s
code-quality (hive_flutter, beta)	1m 22s
test-hive_flutter (beta)	51s
test-hive_generator (stable, ubuntu-latest)	45s
code-quality (hive_generator, stable)	1m 16s
test-hive_generator (stable, windows-latest)	2m 7s
code-quality (hive_generator, beta)	1m 11s
test-hive_generator (beta, ubuntu-latest)	53s
	28m 34s

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