os/bluestore: Add health warning for bluestore fragmentation

os/bluestore: Add health warning for bluestore fragmentation

Changed "bluestore/fragmentation_micros" from quick imprecise to
slow but more representative score.
Introduced config "bluestore_warn_on_free_fragmentation" that controls
when free space fragmentation score becomes a health warning.

Currently calculation of fragmentation score might be non-instant for
severly fragmented disks. It might induce stalls to write IO.
Config value "bluestore_fragmentation_check_period" control score
calculation period.

In future, costly score calculation will be replaced with method that
continously updates score.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>
(cherry picked from commit 72263dbb7984828844c87bdb3b3ff475a68b1ac6)