pushing of sources/artifacts should be imperative when enabled during bst build

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/160)
In GitLab by [[Gitlab user @tristanvb]](https://gitlab.com/tristanvb) on Nov 22, 2017, 11:05

When running bst build and the user has the right to push things, artifacts which were successfully built but not pushed (because of `^C` interruption or aborting because of an orthogonal build failure, or for whatever reason the push failed last time around) are forgotten and never pushed unless explicitly pushed with `bst push`.

This is weird and feels not symmetric with the rest of the build, instead since we already know what needs to be pushed, we should re-add locally cached artifacts which are not present in the remote to the scheduler so they can be pushed if they are not yet pushed.

Start with the bst build and bst push entry points described in the issue, reproducing the interrupted or failed-push cases. Trace how locally cached artifacts are considered after a build and compare that behavior with explicit bst push. Done means eligible cached artifacts are retried during bst build when they are absent from the remote.

Written by the indexing model from the issue text.