* Preparation for having a background checkpoint thread:
frequency of checkpoint taken by that thread is now configurable
by the user: global variable maria_checkpoint_frequency, in seconds,
default 30 (checkpoint every 30th second); 0 means no checkpoints
(and thus no background thread, thus no background flushing, that
will probably only be used for testing).
* Don't take checkpoints in Recovery if it didn't do anything
significant; thus no checkpoint after a clean shutdown/restart. The
only checkpoint which is never skipped is the one at shutdown.
* fix for a test failure (after-merge fix)


include/maria.h:
  new variable
mysql-test/suite/rpl/r/rpl_row_flsh_tbls.result:
  result update
mysql-test/suite/rpl/t/rpl_row_flsh_tbls.test:
  position update (=after merge fix, as this position was already changed
  into 5.1 and not merged here, causing test to fail)
storage/maria/ha_maria.cc:
  Checkpoint's frequency is now configurable by the user:
  global variable maria_checkpoint_frequency. Changing it on the fly
  requires us to shutdown/restart the background checkpoint thread,
  as the loop done in that thread assumes a constant checkpoint
  interval. Default value is 30: a checkpoint every 30 seconds (yes, I
  know, physicists will remind that it should be named "period" then).
  ha_maria now asks for a background checkpoint thread when it starts,
  but this is still overruled (disabled) in ma_checkpoint_init().
storage/maria/ma_checkpoint.c:
  Checkpoint's frequency is now configurable by the user: background thread
  takes a checkpoint every maria_checkpoint_interval-th second.
  If that variable is 0, no checkpoints are taken.
  Note, I will enable the background thread only in a later changeset.
storage/maria/ma_recovery.c:
  Don't take checkpoints at the end of the REDO phase and at the end of
  Recovery if Recovery didn't make anything significant (didn't open
  any tables, didn't rollback any transactions).
  With this, after a clean shutdown, Recovery shouldn't take any
  checkpoint, which makes starting faster (we save a few fsync()s of
  the log and control file).