librbd: expose ContextWQ as pure interface for external implementations

librbd: expose ContextWQ as pure interface for external implementations

Make ContextWQ a pure interface in the public API to allow external modules
to provide custom ContextWQ implementations.

- Move ContextWQ to public header as pure interface (no Boost.ASIO dependencies)
- Extract ASIO implementation into separate AsioContextWQ class
- Add to set ContextWQ at open time
  - rbd_open_with_context_wq()
  - rbd_open_read_only_with_context_wq()
- Add rbd_get_ceph_context() C API to retrieve CephContext from RBD image
- Add rbd_context_complete() C API to complete contexts without full Context
  definition
- Change m_context_wq from unique_ptr to shared_ptr for external ownership
- Install public headers via CMake and include in librbd-devel package

Signed-off-by: Alexander Indenbaum <aindenba@redhat.com>