_sandboxremote.py: Support stdout and stderr digests

Jürg Billeterrequested to merge
juerg/reapi-log into master

Support remote execution log forwarding using stdout and stderr digests instead of only supporting embedded stdout and stderr. This also moves log forwarding from SandboxREAPI to SandboxRemote to avoid duplicating the log with buildbox-run.

The autotools integration test has been extended to check the build log.

See also BuildGrid/buildbox/buildbox-common!222.

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