ObjectStore: add Context management to Transaction

ObjectStore now appends passed contexts in queue_transaction
to the Transaction contexts and uses that to pass into
the virtual queue_transactions.

Signed-off-by: Samuel Just <sam.just@inktank.com>
DBObjectMap, hobject: move hobject->str methods to hobject

Signed-off-by: Samuel Just <sam.just@inktank.com>
common/shared_ptr.hpp: add get_next and expose VPtr and WeakVPtr types

Signed-off-by: Samuel Just <sam.just@inktank.com>
common/: add map_cacher

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: add SnapMapper

Signed-off-by: Samuel Just <sam.just@inktank.com>
test/: add tests for map_cacher

Signed-off-by: Samuel Just <sam.just@inktank.com>
SnapMapper: add adapter for ObjectStore

Signed-off-by: Samuel Just <sam.just@inktank.com>
Makefile: add SnapMapper to osd

Signed-off-by: Samuel Just <sam.just@inktank.com>
SnapMapper: add remove_oid and get_snaps

Signed-off-by: Samuel Just <sam.just@inktank.com>
test_snap_mapper: add tests for remove_oid and check_oid

Signed-off-by: Samuel Just <sam.just@inktank.com>
working