Updates to the bitmap flush/pin logic to prepare for when we support
multiple writers.


storage/maria/ma_bitmap.c:
  Read lock is less bad than write lock.
  Changing bitmap->flushable to a counter, to prepare for when multiple
  writers are allowed on a table.
  Using bitmap->flush_all_requested instead of share->in_checkpoint; the
  latter can be true for the time of a whole checkpoint even though
  the checkpoint is not yet handling our table, or has already handled it,
  so to decrease the number of broadcasts we use a dedicated my_bool
  which is true only when checkpoint is handling this table's bitmap.
  _ma_bitmap_flushable(share,+1) waits for a concurrent _ma_bitmap_flush_all()
  to finish before incrementing non_flushable; without this, with multiple
  writers there may always be one thread making the bitmap unflushable
  and thus checkpoint would stall.
storage/maria/ma_blockrec.c:
  update to new prototype: "flushable is FALSE|TRUE" becomes "add 1|-1 to
  non_flushable".
storage/maria/ma_blockrec.h:
  new prototype
storage/maria/maria_def.h:
  MARIA_FILE_BITMAP::flushable becomes a counter.
  New MARIA_FILE_BITMAP::flush_all_requested.