Merge branch 'ps/object-read-stream'

Merge branch 'ps/object-read-stream'

The "git_istream" abstraction has been revamped to make it easier
to interface with pluggable object database design.

* ps/object-read-stream:
  streaming: drop redundant type and size pointers
  streaming: move into object database subsystem
  streaming: refactor interface to be object-database-centric
  streaming: move logic to read packed objects streams into backend
  streaming: move logic to read loose objects streams into backend
  streaming: make the `odb_read_stream` definition public
  streaming: get rid of `the_repository`
  streaming: rely on object sources to create object stream
  packfile: introduce function to read object info from a store
  streaming: move zlib stream into backends
  streaming: create structure for filtered object streams
  streaming: create structure for packed object streams
  streaming: create structure for loose object streams
  streaming: create structure for in-core object streams
  streaming: allocate stream inside the backend-specific logic
  streaming: explicitly pass packfile info when streaming a packed object
  streaming: propagate final object type via the stream
  streaming: drop the `open()` callback function
  streaming: rename `git_istream` into `odb_read_stream`