Introduce an `overwrite_process` variable that can overwrite the current rule evaluation

The variable overwrite_process could be used, for example to cancel a process to add a Google building entity to the entities table, when there already are OSM buildings that intersect.

The overwrite_process is rather a function of rule-lib than a rule. Any rule can return {"overwrite_process": "CANCEL"}, after which the whole rules evaluation gets cancelled. Maybe we would like to cancel when a building is of a too large size. Maybe we would like to cancel if a certain attribute exists, etc. In the future, maybe we can have other overwrite_processes, such as DELETE, which would stop the whole process and also delete the building from the database. Maybe there are other overwrite_processes that we did not think about now

\fyi @ds @pdlmora @chengzhi