better concurrency properties

• Ensure that Irmin.update and Irmin.merge are atomic.
• Fix Irmin.clone of an empty branch
• Add Irmin.RW.compare_and_test that the backends now have to implement
  to guarantee atomicity of Irmin's high-level operations.
• Add Irmin.Private.Lock to provide per-handler, per-key locking. This
  can be used by backend to implement simple locking policies.
• Add Lwt.t to the return type of Irmin.tag and Irmin.tag_exn
• Do not throw [Not_found]. Now all the _exn function raise Invalid_argument
  (#144)
• Remove Irmin.switch and Irmin.detach
• Add Irmin.history to get the branch history as a DAG of heads (#140).
• Fix the computation of lcas (#160)