Do not reinitialize `Hive` when opening a `BoxCollection` #194

Job	Run time
ensure-codegen	3m 3s
check-score (hive)	1m 34s
code-quality (hive, stable)	1m 17s
check-score (hive_flutter)	2m 0s
code-quality (hive, beta)	1m 11s
ensure-embedme	7s
check-score (hive_generator)	2m 36s
code-quality (hive_flutter, stable)	1m 28s
test-hive (vm, kernel)	1m 1s
code-quality (hive_flutter, beta)	1m 27s
test-hive (chrome, dart2js)	3m 16s
test-hive_flutter (stable)	54s
code-quality (hive_generator, stable)	1m 17s
test-hive (chrome, dart2wasm)	2m 59s
test-hive_flutter (beta)	55s
code-quality (hive_generator, beta)	1m 18s
test-hive_generator (stable, ubuntu-latest)	53s
test-hive_generator (stable, windows-latest)	2m 16s
test-hive_generator (beta, ubuntu-latest)	51s
	30m 23s

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