Keep statistics on number of BIO_READ, BIO_WRITE, BIO_DELETE and BIO_FLUSH

Keep statistics on number of BIO_READ, BIO_WRITE, BIO_DELETE and BIO_FLUSH
requests as well as number of activemap updates.

Number of BIO_WRITEs and activemap updates are especially interesting, because
if those two are too close to each other, it means that your workload needs
bigger number of dirty extents. Activemap should be updated as rarely as
possible.

MFC after:	1 week