os/bluestore: use string_view in BlueFS

os/bluestore: use string_view in BlueFS

this improves the performance of lookup. when it comes to mutation
operations, std::map<> always require an instance of string, but
it only require a single instance of string for each mutation operation
in general, so this does not incurs performance regression due to
multiple copies of the same string_view object.

Signed-off-by: Kefu Chai <kchai@redhat.com>