archive: Implement ApplyLayer directly

Rather than calling out to tar we use the golang tar parser
to directly extract the tar files. This has two major advantages:

1) We're able to replace an existing directory with a file in the
   new layer. This currently breaks with the external tar, since
   it refuses to recursively remove the destination directory in
   this case, and there are no options to make it do that.

2) We avoid extracting the whiteout files just to later remove them.
Fix change detection when applying tar layers

The default gnu tar format has no sub-second precision mtime support,
and the golang tar writer currently doesn't support that either.
This means if we export the changes from a container we will not
get zeron in the sub-second precision field when the change is applied.

This means we can't compare that to the original without getting a
spurious change. So, we detect this case by treating a case where the
seconds match and either of the two nanoseconds are zero as equal.
Re-enable TestApplyLayer

With the previous two changes we now pass this test.
ApplyLayer: Fix TestLookupImage

The TestLookupImage test seems to use a layer that contains
/etc/postgres/postgres.conf, but not e.g. /etc/postgres.

To handle this we ensure that the parent directory always
exists, and if not we create it.
Merge pull request #3207 from alexlarsson/fix-applylayer

Re-enable TestApplyLayer and make it work