relayd: replace uses of block FDs by the fs_handle interface

Replace all usage of "raw" block-device file descriptors for
relay_streams, viewer_streams, and index files by the fs_handle.

Wrappers are introduced for read, write, seek and truncate operations
in order to reduce code duplication as all uses of fs_handles implies
getting an fd, using it, and putting it back. Those operations allow
the fd-tracker to suspend and restore fs_handles as needed.

The stream_fd util is eliminated as it is completely replaced by
the fs_handle interface.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iedff88d27aeba3891d4e8818b9e08e4b16a927cc