Support Asynchronous Writing Datastores

[aschmahmann]

Per #137 we would like to support Datastores that internally have asynchronous writes. We are doing this by adding a Sync(prefix ds.Key) function to the Datastore interface.

The plan is: 1) Get this PR reviewed for a sanity check 2) Get the datastore PRs submitted and reviewed and ready to be accepted (their go.mod files will reference this PR's branch)

• [x] https://github.com/ipfs/go-ds-measure/pull/23
• [x] https://github.com/ipfs/go-ds-s3/pull/46
• [x] https://github.com/ipfs/go-ds-redis/pull/6
• [x] https://github.com/ipfs/go-ds-leveldb/pull/36
• [x] https://github.com/ipfs/go-ds-flatfs/pull/60
• [x] https://github.com/ipfs/go-ds-badger/pull/77
• Note: If some of the datastores are proving tricky to write a Sync command for we may not block for them all to be approved before we continue. 3) Merge this PR 4) Update and merge the other PRs 5) Update some areas of go-ipfs to use the Sync() command on their datastores 6) Switch the default datastore in go-ipfs to be an asynchronous one (i.e. Badger with Async writes enabled)

[bonedaddy]

After this PR is merged can we have a version bump according to semver practices since this is a breaking change to the Datastore interface?

[Stebalien]

After this PR is merged can we have a version bump according to semver practices since this is a breaking change to the Datastore interface?

We will bump the minor version because the package version is sub-zero.

[bonedaddy]

After this PR is merged can we have a version bump according to semver practices since this is a breaking change to the Datastore interface?

We will bump the minor version because the package version is sub-zero.

Won't break go get -u=patch so :rocket:

[Stebalien]

LGTM but let's merge this tomorrow. Hitting merge then going to bed is a bad idea (I mean, it's one of my favorite bad ideas but I think it's time to call it a day).