changes to how synchronous checkpoint requests are executed.
changes to how the background LRD flushing thread refrains from using
all resources. See more comments for each file.


storage/maria/checkpoint.c:
  I was not happy that checkpoint requests which want to know the success/error
  of their executed request, get inaccurate information in case of error
  (no error string etc). Instead of implementing a more complete communication
  protocol between requestor and executor, I make the requestor do the
  execution itself. I call this a synchronous checkpoint.
  For asynchronous checkpoints (requestor does not want to know success/error,
  does not want to wait for completion), no change, checkpoint is executed
  by the background thread.
  Comments, constants, mutex usage fixes.
storage/maria/checkpoint.h:
  new prototype of "API" (the calls exposed by the checkpoint module)
storage/maria/least_recently_dirtied.c:
  A better solution than sleeping one second after flushing a piece of the
  LRD: instead we pthread_yield(). Hopefully this will slow down the background
  thread (avoiding it using all the disk's bandwidth) if there are other threads 
  competing, and will not slow it down if this thread is alone (where we do
  want it to run fast and not do useless sleeps).
  This thread will probe for asynchronous checkpoint requests every few seconds.