BufferDesc *BufferAlloc(
                        Relation r, ForkNumber f,
                        BlockNumber n, bool *found)

• used by ReadBuffer to find a buffer for (r,f,n)
• if (r,f,n) already in pool, pin it and return descriptor
• if no available buffers, select buffer to be replaced
• returned descriptor is pinned and marked as holding (r,f,n)
• does not read; ReadBuffer has to do the actual I/O