dvc: make flufl.lock opt-in and use zc.lockfile

[efiop]

As it turned out (see issue numbers down below), we can't really take
hardlinks for granted, so flufl.lock is not a panacea for all
filesystems. Considering that the vast majority of filesystems that our
users use support zc.lockfile(flock-based) and it has benefits like
more reliable mechanism, auto-delete when process exits, more sturdy
implementation, etc, it makes more sense to bring it back and use by
default again. For filesystems that don't support flock(), users will
be able to manually enable flufl.lock use through the config option.
It would be ideal if we could auto-detect that flock is not supported,
but in the real world, it turned out to be non-trivial, as it might hang
forever in a kernel context, which makes the implementation way too
complex for our purposes. So what we're doing instead is showing a
message before locking with zc.lockfile that, under normal
circumstances will disappear once the lock is taken or failed, otherwise
it will point users to the related documentation where they can learn
about how to opt-in for flufl.lock.

Fixes #2831
Fixes #2897
Fixes #2944
Related #2860

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iterative/dvc.org#858

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[efiop]

Note to self: no need to update this in our conda package, as I've left zc.lockfile there in preparation for this PR.

[Suor]

I would name it explicitly, this is shadowing the intent and tying up the interfaces.

[Suor]

Looks good.

[ghost]

looks good to me, @efiop ;

[shcheklein]

@efiop @jorgeorpinel I've just put a few minor UI comments, please take a look and improve/fix if needed.

[shcheklein]

@efiop btw, just curious, how does it affect dvc get (DVC API?) - is this config supposed to be saved in Git? should we recommend using --local and/or --global for this option?

[efiop]

@shcheklein dvc get is not using repo locks. The only thing I am worried about is analytics lock (for the user_id file), but that one runs after the command and is located in ~, which is less common to have some weird fs, so the user won't notice it most of the time. Will give it a closer look, but it could wait until someone complains about it.

[pared]

@efiop does config have verify argument? When I run any dvc command, I get ERROR: unexpected error - init() got an unexpected keyword argument ' verify' in the log.