delete WAL files in descending order during walRollback

    * search for the last entry in the contiguous range of valid WAL from the last pos fsynced as firstTrial
    * find the last entry before the last pos fsynced backwards as second trial
    * reserve sufficient space for computing CRC32 checksum, esp. of WAL record body
    * rebuild meta info to resolve potential misalignment between lists of meta and actual log files
    * retract commitIndex and appliedIndex to lastLogIndex if needed
    * put an upper size limit on possibly corrupted WAL range to be recovered