dvc: .dvc/lock and sqlite lock on NFS and CIFS #1918
Description
As it turns out, regular locks can't be relied on when dvc is running on NFS or CIFS. See #1823 and https://discordapp.com/channels/485586884165107732/485596304961962003/570270243964846081 .
There are workarounds such as adding proper mnt options or moving dvc project outside with an external cache directory at NFS/CIFS mount, but it would be great to mitigate such issues in general. Here are a few ways to go about it:
One way to go about it is to use git-like locks (which, if I recall correctly, are using symlinks as an atomic way lock/unlock a file). In that case, we would have to use an unlocked sqlite db relying on our .dvc/lock (or maybe introduce a separate special lock specifically for a db).