Simplify the nano::ledger::successor function to make its implementation more obvious. #1431

Job	Run time
macOS [UBSAN] [lmdb \| clang]	32m 24s
Linux [UBSAN] [lmdb \| clang]	38m 36s
macOS [ASAN] [lmdb \| clang]	32m 1s
Linux [ASAN] [lmdb \| clang]	14m 55s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	26m 6s
macOS [TSAN] [lmdb \| clang] (Errors Ignored)	32m 5s
Linux [TSAN] [lmdb \| clang] (Errors Ignored)	15m 0s
macOS [UBSAN] [rocksdb \| clang]	39m 11s
macOS [ASAN] [rocksdb \| clang]	41m 53s
macOS [TSAN] [rocksdb \| clang] (Errors Ignored)	50m 23s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	25m 33s
Linux [UBSAN] [rocksdb \| clang]	41m 36s
Linux [ASAN] [rocksdb \| clang]	13m 28s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	24m 18s
Linux [TSAN] [rocksdb \| clang] (Errors Ignored)	12m 41s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	13m 28s
	7h 33m 38s

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