Safe spool

Safe spool is C++ library suitable for realtime queueing, persistent queues,
spooling binary items, while giving guarantees for persistence. It has been
optimized for putting things into the queue.

It works by always writing the items to queue into a file, and the letteing
a background thread forward the items. The queue file is always written and
read sequentially which improves performance a lot. This sounds impossible
but it is not. When the queue file has grown to a (configurable) size the
library will start writing new items to an overflow file. When the background
forwarding thread has forwarded all the items that are in the normal file
but not in the overflow file if will switch to overflow file to the normal
file and continue.

Things to worry about:
  - If you forward-implementation pushes the items to the network then 
    if the incoming rate exceeds the rate that you can push it through
    the network the queue file will grow and grow...
  - If you need an absolute guarantee that things have been written to
    the disc you need to fiddle with the O_DSYNC options to open() and
    create, or put the queue files on a "sync" mounted filesystem. For
    high loads don't do this unless you use a journaled filesystem
    (unless you like to make your disc sound like a chainsaw...)
  - The maximum throughput is limited by your disc bandwidth divided by 3.
  - Sometimes items are sent to your forwarding function more than once.
    Deal with it.

Performance:
  My system has no problems with queueing 23MB/sec