[crimson] bluestore in an alien world

hi Chunmei, i am reviewing your change of https://github.com/ceph/ceph/compare/master...liu-chunmei:ceph_seastar_alie…. it looks good in general. i think the simplest way to co-locate different versions of alien-common, ceph-common and crimson-common is to introduce different namespaces. because we need to have alien-common and crimson-common in the same binary, and to have all of these three versions in the same repository. but this divergence concerns me, as it introduces yet another condition in the shared infrastructure in our code base. and in the long run, this #ifdef won't go away if we want to go this way, so i need to at least give it a try. what is "it"? to port rocksdb to seastar. as seastar offers "seastar::thread" which makes it relative simpler to wrap the blocking calls with ucontext. and rocksdb offers a abstraction machinery allowing one to port it to a new platform. and seastar is a "platform" to some degree, i'd say. will update you guys with my progress and findings. -- Regards Kefu Chai