crimson/os/seastore: implement generational GC

crimson/os/seastore: implement generational GC

Place extents into the dedicated RecordSubmitter by their data-category
and reclaimed-count. Segments of different data-category or
reclaimed-count should have different locality in the access patterns,
which is the foundation to form a desired bimodal distribution of
segment utilizations, so that GC can be more efficient.

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>