Exit migration if existing rocksdb ledger found #2415

Job	Run time
Linux [UBSAN] [lmdb \| clang]	38m 2s
macOS [UBSAN] [lmdb \| clang]	38m 0s
Linux [ASAN] [lmdb \| clang]	38m 42s
macOS [ASAN] [lmdb \| clang]	30m 41s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	38m 12s
macOS [TSAN] [lmdb \| clang]	33m 31s
Linux [TSAN] [lmdb \| clang]	41m 25s
macOS [UBSAN] [rocksdb \| clang]	44m 30s
Linux [LEAK] [lmdb \| clang]	40m 1s
macOS [ASAN] [rocksdb \| clang]	38m 11s
Linux [UBSAN] [rocksdb \| clang]	47m 18s
Linux [ASAN] [rocksdb \| clang]	52m 28s
macOS [TSAN] [rocksdb \| clang]	54m 0s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	52m 12s
Linux [LEAK] [rocksdb \| clang]	52m 38s
	10h 39m 51s

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