memstore: PageSetObject for MemStore integration

memstore: PageSetObject for MemStore integration

adds config variable memstore_page_set=true to switch between the
PageSet and bufferlist implementations for object data

benchmark results:

test command:
./ceph_objectstore_bench --osd_objectstore=memstore --osd_data=/home/casey/osbench --size=1g --block-size=X --threads=Y --repeats=32

test hardware: 2x Intel(R) Xeon(R) CPU E5-2620 0 @ 2.00GHz (12 cores/24
threads), 64GB memory

bufferlist (sequential write bandwidth in MB/s):

threads	bs=4k	bs=64k	bs=256k	bs=1m	bs=4m
1	x	63	340	6177	81861
8	x	17	304	5910	87708
16	x	x	320	5959	86578
        x: bandwidth approached 0 as size grew

PageSet (sequential write bandwidth in MB/s):

threads	bs=4k	bs=64k	bs=256k	bs=1m	bs=4m
1	197	2122	3085	4471	4712
8	626	8808	15518	20389	18926
16	726	9581	18997	22330	21983

bufferlist performs extremely well in constrained workloads (large block
size and total file size below ~10G), because its writes are zero-copy.
but it breaks down at smaller block sizes due to the linear property of
the bufferlist

PageSet trades the zero-copy property for a logarithmic data structure
in order to achieve more reliable performance across file and block
sizes, in addition to better thread scaling

note: because each PG operates on a different collection and uses the
Sequencer to serialize the operations therein, we're not likely to see
the benefits from multilpe threads in the OSD

Signed-off-by: Casey Bodley <cbodley@redhat.com>