Everytime the purge check was running, a new segment was being added.
This meant the list of almost-empty files in the HH directories would
grow continually.
If the hinted handoff segment is corrupt, the size read could be
invalid and attempting to create a slice using that size causes
a panic. Ideally, we'd have a checksum on the seqment record but
for now just return an error when the size is larger than the
segment file.
Fixes#3687
The advance function was not writing the updated position in the
queue until after the next advance call was called. Resulted in
the very last message would get replayed on restart each time.
Edd Robinson
Philip O'Toole
Jason Wilder
Joseph Crail