feat: updated DataStore [2/n]

[gmaclennan]

From #139:

A DataStore should be an abstraction over a particular "Mapeo Store", e.g. a collection of hypercores that store a particular datatype. Responsibilities:

• Write a document to the write core of the store.
• Read a document based on versionId.
• Manage read indexing and return current indexing state.
• Handle decoding/encoding of data

With this we can isolate the boundary with hypercore / core store to this module (apart from within the blob store), and largely isolate indexing: The write method can resolve when the document is indexed.

const store = new DataStore({
  coreManager,
  namespace: 'auth' | 'data',
  // DataStore will consider a block "indexed" once this function resolves.
  indexDocs: (blocks: Buffer[]) => Promise<void>
})

// Guarantee that `doc` is indexed once this resolves
store.write(doc: Exclude<MapeoDoc, 'versionId'>): Promise<MapeoDoc>

store.read(versionId: string): Promise<MapeoDoc>

store.getIndexState() // return whether is currently indexing, and estimate of indexing remaining to be done

store.on('index-state') // same as above.

[gmaclennan]

Now largely complete, will be best to review once all items in #139 are done, because this might still change.

[achou11]

couple of minor suggestions. also need to remove a test.solo call to see if all the added tests pass, but otherwise seems good to me

Is the plan to eventually use this as the datastore implementation (currently named to avoid collision with existing one)

[achou11]

extract these events to a constant? i.e. const LISTENER_EVENTS = ['newListener', 'removeListener'] (you can come up with a better variable name 😄 )

[gmaclennan]

I'd prefer not to because I'd like this to be explicit in the code, rather than the indirection of a constant - we'd end up naming it "newListenerAndRemoveListenerEventNames". Making is a constant might be a slight performance improvement, but this is not somewhere that is a bottleneck (events are added rarely).

[gmaclennan]

Rebased onto changes in #140

[gmaclennan]

Merged in #149