Remove the 1st-class module API and expose server-side transaction in the REST API

• Expose the Git compression level (#104, #298 by @samoht)
• Add an optional config argument to all the backend's config
  functions. This allow the backends to composed more easily. (initial
  patch by @nasrallahmounir, integration by @samoht)
• Add signatures for immutable link store, to store links between
  keys: Irmin.LINK and Irmin.LINK_MAKER. Add Irmin_mem.Link and
  Irmin_fs.Link which implement Irmin.LINK_MAKER in these backends
  (initial patch by @nasrallahmounir, integration by @samoht)
• Add signatures for raw values (ie. whose values are of type
  Cstruct.t): Irmin.RAW and raw store maker: Irmin.AO_MAKER_RAW
  (initial patch by @nasrallahmounir, integration by @samoht)
• Expose Irmin.Hash.digest_size (initial patch by @nasrallahmounir,
  integration by @samoht)
• Expose /view to the REST API (#292, by @samoht)
• Expose Irmin.Private.merge_node (#292 by @samoht)
• Change the JSON stream API, which requres ezjsonm.0.4.2. (#266, #269,
  #273 by @samoht)
• Fix a race when a lot of processes are trying to add a watch at the
  same time. (#270, #271, by @samoht)
• Expose Irmin_git.Irmin_value_store functor. This provides the
  Irmin Contents/Node/Commit APIs on top of a Git-type store. This is
  useful for backends that want to store data using the Git object
  format, to be able to sync with Git, but without using Git's
  filesystem layout and locking. (#268 by @talex5)
• Remove the first-class module API. It's confusing to duplicate the API
  (#293, by @talex5)