This improves readability.

This adds the batch() context manager.

This allows the sandbox to output messages using the plugin id.

This allows batching of all sandbox commands aross prepare() and
assemble().

Execute batched commands in a single shell script.

Command batching

Closes #675

See merge request !915

There is nothing in ArtifactCacheSpec that's actually specific to
artifacts, so I've made it a CAS class so we can use the same
spec for remote execution.

Since the artifact cache and remote execution share the same
local CAS store, they should share the same CASCache object. Moving
this into context allows us to do this.

Uses the new config options introduced earlier to specify remote
exec storage and execution service.

Tests that we get a sensible error message when we specify incomplete
or wrong information in the configuration for remote execution.

The port number must come after the hostname and before the path.
The path is ignored by CASCache anyway, so including it is misleading.

Split remote execution from artifact cache

Closes #750

See merge request !946