lib: add “wait-for” jobs

The idea is tocheck a target as a “condition” when the condition becomes
true another `todo` job gets declared.
test: run “wait-for” jobs
test: add `--interactive` option (run any command)

The goal is to hide a temporary persistence file.
Execution_engine: log changes (“wait-for” jobs)
Merge branch 'wait_for_todos' into 'master'