Fix possible race condition in open_blkfile (GitHub issue #10)

There was a possible race condition in open_blkfile: bitcoind may have
written to the last file by the time we open the next one.  As per
  https://github.com/jtobey/bitcoin-abe/issues/10 we should:
    1. Check the presence of a next file
    2. Read current blockfile
    3. If next file was there in 1, move on to it, else return.