Overhaul of `online_reps` class #2540

Job	Run time
macOS [UBSAN] [lmdb \| clang]	30m 11s
macOS [ASAN] [lmdb \| clang]	25m 33s
macOS [TSAN] [lmdb \| clang]	21m 2s
macOS [UBSAN] [rocksdb \| clang]	36m 53s
macOS [ASAN] [rocksdb \| clang]	35m 53s
macOS [TSAN] [rocksdb \| clang]	45m 14s
Linux [UBSAN] [lmdb \| clang]	37m 3s
Linux [ASAN] [lmdb \| clang]	36m 50s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	39m 7s
Linux [TSAN] [lmdb \| clang]	39m 1s
Linux [LEAK] [lmdb \| clang]	37m 31s
Linux [UBSAN] [rocksdb \| clang]	46m 50s
Linux [ASAN] [rocksdb \| clang]	50m 21s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	52m 1s
Linux [LEAK] [rocksdb \| clang]	53m 55s
	9h 47m 25s

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