Buffer Manager

Fix, unfix and fix_new (allocate and fix new pg)
Aries uses steal policy - uncommitted writes may be output to disk (contrast with no-steal policy)
Aries uses no-force policy (updated pages need not be forced to disk before commit)
dirty page: buffer version has updated not yet reflected on disk
- dirty pages written out in a continuous manner to disk

Fix, unfix and fix_new (allocate and fix new pg)