Read correct number of messages from end of file

[leonzz]

Directly read from file instead of the bufio.Reader to prevent wrong data from buffer.

Also with some trivial improvements on checkes:

• deleted the if expectedBytesIncrease > d.maxBytesDiskSpace check from freeDiskSpace because the same check has been done already in checkDiskSpace.
• in constructor, ensure disk size limit is larger than a max sized file + metadata file.

[kev1n80]

LGTM. Thank you for implementing the solution for the negative depth error and adding improvements!