Exit migration if existing rocksdb ledger found #2413

Job	Run time
Linux [UBSAN] [lmdb \| clang]	39m 16s
macOS [UBSAN] [lmdb \| clang]	33m 32s
Linux [ASAN] [lmdb \| clang]	37m 53s
macOS [ASAN] [lmdb \| clang]	31m 22s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	38m 45s
macOS [TSAN] [lmdb \| clang]	32m 20s
Linux [TSAN] [lmdb \| clang]	39m 29s
macOS [UBSAN] [rocksdb \| clang]	43m 41s
Linux [LEAK] [lmdb \| clang]	37m 24s
macOS [ASAN] [rocksdb \| clang]	38m 22s
Linux [UBSAN] [rocksdb \| clang]	46m 37s
macOS [TSAN] [rocksdb \| clang]	51m 53s
Linux [ASAN] [rocksdb \| clang]	54m 39s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	55m 50s
Linux [LEAK] [rocksdb \| clang]	53m 48s
	10h 34m 51s

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