* exp: allow fetching from arbitrary executor by git URL
* exp: write info for running executors to .dvc/tmp/exp/run
currently only includes PID and git fetch URL
* exp: refactor stash/queue cleanup
* exp: add ability to collect dict of currently running experiments
* exp show: display run/queue state in the table
* exp run: unlock main repo while running tmpdir executors
* add tests for new exp show behavior
* fix workspace checkpoint run handling
* fix issue where running stash revs were shown as queued
* use "running"/"queued"
* add executor (location) column
* compare: add TabularData.is_empty
* use td.is_empty to drop state/executor
* use "workspace" and "temp" as executor names