set rwlock on all dvcfiles when load/dump-ing

[skshetry]

We support running repro/run in parallel, so, there might be some chance of things getting overwritten between the load-serialize-dump process, creating a "last writer wins" situation.

[efiop]

Related #3653

[skshetry]

Looks like we don't completely allow running parallel execution, as we lock the repo most of the times and only unlock when the command is really running.

dvc/dvc/stage/run.py

Lines 37 to 38 in afbb3a0

	@unlocked_repo
	def cmd_run(stage, *args, **kwargs):

As the collection of stages happen when the repo is locked in, only one process of dvc can really
be running at that point of time. So, I'm reducing it's priority for now and removing from 1.0 milestone.

But, if we later come to remove that lock or make amends to it to allow parallel execution at all times, this will be p0-critical.

[skshetry]

Closing as stale.