concurrency improvement in vmtrim - the vmtrim thread temporarily yields the lock during the saving of   a tile, while noting in MT_mmap_busy which heap it is busy saving. - MT_mmap_new (BATnew) is thus no longer held up by vmtrim activity - MT_mmap_del (BATdestroy) has to be a little more careful.. if the   vmtrim thread is saving a tile of the heap (MT_mmap_busy) that is   the target of deletion, it must wait for it to finished.   Otherwise it can just go on.
concurrency improvement in vmtrim
- the vmtrim thread temporarily yields the lock during the saving of
  a tile, while noting in MT_mmap_busy which heap it is busy saving.
- MT_mmap_new (BATnew) is thus no longer held up by vmtrim activity
- MT_mmap_del (BATdestroy) has to be a little more careful.. if the
  vmtrim thread is saving a tile of the heap (MT_mmap_busy) that is
  the target of deletion, it must wait for it to finished.
  Otherwise it can just go on.

the previous hard locking of all MT_mmap structures during a vmtrim could
previously seriously hold up progress in other threads.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb