Avoid over scanning data directories in LogFile.verify()

We currently list the data directory for every REMOVE record in the file in LogFile.verify() - this can get very expensive during startup when we call LogTransaction.removeUnfinishedLeftovers(). In LogRecord.getExistingFiles(Set<String> absoluteFilePaths) we also fully parse the file name of the sstables found, here we only need to prefix match.