Avoids an expensive lookup of the previous snapshot.

Local caches apparently require an exclusive lock, effectively
preventing concurrency -- it is still to be seen if the repository
lock has the same effect though.

V2

See merge request !1

This way we can detect failure of dump / restore commands when used
with the 'pipe' handler (when we're using shell pipes for composition).

Clear separation between configuration and its parsed results, which
are now maintained in the RuntimeContext to kept them consistent
within backup/restore jobs.

This makes it easier to generate application-level atom paths that are
manageable. Datasets are given unique IDs at generation time.

Use LZ4 by default.

It may be set on the atom instead.

Paths (for atoms and datasets) only exist within the API itself now.

Try to make things simpler and more clear. Specifically:

* clarify the semantics of Source, Dataset and Atom
* separate the namespace of Atom paths from the Restic namespace
* simplify the command execution by delegating pipes to /bin/sh
  for efficiency

There's still a bunch to do.

This is necessary in consequence of issue #2.

Rewrite all internal references to the old package repository path.

Send SIGUSR1 to the agent binary.

Support both single-object and lists when reading handlers and sources
from YAML files in a directory.

When running commands, drop locale- and restic-related environment
variables.

Prevents interactions from the local environment where the test is running.

If disabled, we won't run "restic prune" on the remote
repository. Allows support of append-only repositories.

Fixes issue #1.

Restic 0.9.3 fails when --stdin-filename contains slashes:

repository 954a4e61 opened successfully, password is correct
error: file does not exist
Fatal: unable to save snapshot: snapshot is empty