Enhancement: Rework archiver code

The core archiver code and the complementary code for the `backup` command was
rewritten completely. This resolves very annoying issues such as 549. The first
backup with this release of restic will likely result in all files being
re-read locally, so it will take a lot longer. The next backup after that will
be fast again.

Basically, with the old code, restic took the last path component of each
to-be-saved file or directory as the top-level file/directory within the
snapshot. This meant that when called as `restic backup /home/user/foo`, the
snapshot would contain the files in the directory `/home/user/foo` as `/foo`.

This is not the case any more with the new archiver code. Now, restic works
very similar to what `tar` does: When restic is called with an absolute path to
save, then it'll preserve the directory structure within the snapshot. For the
example above, the snapshot would contain the files in the directory within
`/home/user/foo` in the snapshot. For relative directories, it only preserves
the relative path components. So `restic backup user/foo` will save the files
as `/user/foo` in the snapshot.

While we were at it, the status display and notification system was completely
rewritten. By default, restic now shows which files are currently read (unless
`--quiet` is specified) in a multi-line status display.

The `backup` command also gained a new option: `--verbose`. It can be specified
once (which prints a bit more detail what restic is doing) or twice (which
prints a line for each file/directory restic encountered, together with some
statistics).

Another issue that was resolved is the new code only reads two files at most.
The old code would read way too many files in parallel, thereby slowing down
the backup process on spinning discs a lot.

https://github.com/restic/restic/issues/549
https://github.com/restic/restic/issues/1286
https://github.com/restic/restic/issues/446
https://github.com/restic/restic/issues/1344
https://github.com/restic/restic/issues/1416
https://github.com/restic/restic/issues/1456
https://github.com/restic/restic/issues/1145
https://github.com/restic/restic/issues/1160
https://github.com/restic/restic/pull/1494