Optionally process transactions in the background

[DavidMStraub]

The transactions endpoint is used for raw database transactions, which is used in particular by the sync addon (but is also required for the future revert option #511). Right now, this is done synchronously. But it includes updating the full-text and semantic search indexes. For a large number of transactions (e.g. syncing very out-of-sync trees), this can take very long, in particular with semantic search enabled, so we should process it in the background. For backwards compatibility, background processing should be optional.