upper-level WiredTiger code is reaching into the block manager

There are a couple of places where the upper-level WiredTiger code is reaching into the block manager: conn_dhandle.c looks at btree->bm->map, bt_cursor.c looks at btree->bm->block->fh->size.

The hope is the block manager can be ported to something other than a POSIX filesystem if necessary, so we'd like to avoid this kind of access.