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Add test to ensure that Delete returns no error for missing keys

Fix typo in sync/sync.go

Add the generated flatfs stub, since it cannot be auto-generated

support flatfs fuzzing

Adds a go-fuzz entrypoint for fuzzing datastore transactions for crashes.

chore(deps): bump go.uber.org/multierr from 1.4.0 to 1.5.0

Bumps [go.uber.org/multierr](https://github.com/uber-go/multierr) from 1.4.0 to 1.5.0.
- [Release notes](https://github.com/uber-go/multierr/releases)
- [Changelog](https://github.com/uber-go/multierr/blob/master/CHANGELOG.md)
- [Commits](https://github.com/uber-go/multierr/compare/v1.4.0...v1.5.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

feat(mount): don't give up on error

/test: fix bad ElemCount/10 lenght (should not be divided)

Fix test log message about number of values put

test suite: Add ElemCount to control how many elements are added.

fix: avoid filtering by prefix unless necessary

feat: add upper-case keys at a known prefix

I need this for testing flatfs.

There's no need to filter by the / prefix.

Instead of giving up when a single datastore fails, operate on all datastores
first. That way:

* If one datastore doesn't support disk usage, we can at least return our best
  estimate for the ones that do.
* If one datastore fails to sync, we won't fail to sync the others.
* If one datastore refuses to close, we'll close the others.

Etc.

Also:

* Drop xerrors. We already need go 1.13 elsewhere.
* Use uber's multierr. It's much simpler than hashicorps (and is already a
  dependency through zap, fx, etc.).

test(suite): add a bunch of prefix tests for the new behavior

Only count a key as an ancestor if there is a separator

This should have already been committed...

Also make sure to clean and normalize keys before using them as prefixes.

BREAKING CHANGES:

* `myds.Query(Query{Prefix:"/foo"})` will no longer match "/foobar" (or even
  "/foo"). This is usually what the user expects, we had a tendency to normalize
  "/foo/" to "/foo" (when we clean keys), and many datastores can't efficiently
  search for prefixes that aren't on path-boundaries anyways.
* Given a mount datastore with mounts `["/foo", "/"]`, `myds.Put("/foo", "bar")`
  will put the value to the datastore mounted at "/", not "/foo", as the key "/"
  and "" usually doesn't make sense.

While technically breaking, these changes are much more likely to fix bugs than
they are to break things.