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query: make datastore ordering act like a user would expect

This applies orders hierarchically. Unfortunately, the interfaces had to be
changed significantly to get this to work.

For example `[OrderByValue{}, SortByKey{}]` *used* to re-sort by key, throwing
away the value sort. Now, this acts like a normal database and `SortByKey` will
only sort within equivalent values.

delayed: implement io.Closer and export datastore type.

That way, we can return it directly.

split the datastore into a read and a write interface

This way, we can abstract over transactions, datastores, and batches when we
just need to write to the datastore.

gx publish 3.4.1

Describe behavior of Batching datastores

handle concurrent puts/deletes in BasicBatch

(also, clear datastore between tests)

add a GetSize method

We can also add this as an extension but it's simple enough I'd like to add it
directly to the Datastore.

Use-case: The blockstore now has a GetSize and bitswap uses it for packing
multiple blocks into a single message. Unfortunately, this means that bitswap is
now calling blockstore.GetSize which calls datastore.Get which tends to be more
expensive than datastore.Has.

Allow ds.NewTransaction() to return an error

Add support for querying entry expiration

improve testing

Add TTL and Transactional interfaces

This reverts commit 8cca1683a03aace2013e48679302100b3580d2ca.