[BUG] Lock may encounter in scan

[birdstorm]

Describe the bug

when we use snapshot.scan, we may find locks that are not resolved.

WARN 2019-08-09 11:01:03 com.pingcap.tikv.operation.iterator.ScanIterator.resolveCurrentLock(120) | resolve current key error locked {
primary_lock: "mDDLJobAd\377dIdxList\377\000\000\000\000\000\000\000\000\367\000\000\000\000\000\000\000L"
lock_version: 410339157315682305
key: "mDDLJobHi\377story\000\000\000\374\000\000\000\000\000\000\000H"
lock_ttl: 3199
}