crimson: add FuturizedCollection

crimson: add FuturizedCollection

we will need to talk to BlueStore, and currently `ceph::os::Collection`
only works with `CyanStore`, so a wrapper around `ceph::os::Collection`
is added. we can subclass it to adapt to CyanStore and to BlueStore.

the underlying concrete `FuturizedStore` classes are responsible to
cast `FuturizedCollection` to their own `Collection` type if they
want to access the interface not exposed by `FuturizedCollection`.

Signed-off-by: chunmei Liu <chunmei.liu@intel.com>