Avoid holding long transactions in `json_handler::blocks_info` #409

Job	Run time
Linux [UBSAN] [lmdb \| clang]	38m 13s
Linux [ASAN] [lmdb \| clang] (Errors Ignored)	37m 18s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	40m 19s
Linux [TSAN] [lmdb \| clang] (Errors Ignored)	38m 58s
Linux [UBSAN] [rocksdb \| clang]	44m 21s
Linux [ASAN] [rocksdb \| clang] (Errors Ignored)	48m 44s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	50m 22s
Linux [TSAN] [rocksdb \| clang] (Errors Ignored)	1h 33m 50s
	6h 32m 5s

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