Use journaling and implement cache for async backends internally.

[james-pre]

Currently, to use sync functions with an async backend, one must use AsyncMirror .

I propose (in conjunction with #338) async backends be updated to work similar to this example:

class AsyncFS <SyncBackend, AsyncBackend> {
  //pretty much AsyncMirror, internally handle sync/async ops but with backends
}

IndexedDB:

import AsyncFS
import InMemoryBackend

class IndexedDBBackend implements Backend {
    ...
}

export class IndexedDB extends AsyncFS<InMemeoryBackend, IndexedDBBackend> {
    ...
}

With this, the end user can just treat it as synchronous.

Note that this should be worked on after #341

[james-pre]

May or may not be done as part of #338