crimson/common,cmake: use seastar API Level 5

crimson/common,cmake: use seastar API Level 5

* cmake: bump Seastar_API_LEVEL to "5"
* errorator.h: add invoke() methods to futurize variants, as in seastar
  API level 5, the future's store type is now either monostate or
  a single T, instead of tuple. so it'd be simpler to integrate
  with seastar with invoke() semantic without the boxing-unboxing
  dance.
* errorator.h: drop apply() methods, as they are not used anymore.
* errorator.h: rename and revise tuple2future. as the store type
  is not tuple now, the trait's name is changed accordingly to
  match seastar::internal::future_stored_type

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