Overhaul of `online_reps` class #2545

Job	Run time
Linux [UBSAN] [lmdb \| clang]	36m 59s
Linux [ASAN] [lmdb \| clang]	39m 22s
macOS [UBSAN] [lmdb \| clang]	32m 51s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	38m 40s
macOS [ASAN] [lmdb \| clang]	25m 46s
Linux [TSAN] [lmdb \| clang]	38m 34s
macOS [TSAN] [lmdb \| clang]	29m 29s
Linux [LEAK] [lmdb \| clang]	39m 56s
macOS [UBSAN] [rocksdb \| clang]	37m 38s
Linux [UBSAN] [rocksdb \| clang]	47m 32s
macOS [ASAN] [rocksdb \| clang]	36m 46s
Linux [ASAN] [rocksdb \| clang]	50m 39s
macOS [TSAN] [rocksdb \| clang]	45m 57s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	52m 46s
Linux [LEAK] [rocksdb \| clang]	52m 0s
	10h 4m 55s

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