Generalize transaction listeners for tx-report -> remote notifications

[metasoarous]

In line with #13, we'll have to generalize the ability to add transaction listeners to both Datomic and DataScript, such that tx-report datoms get translated and sent out to all subscribed remotes. This starts to beg the question of how those subscriptions are (generally speaking) managed (see #10), and how we apply authorization and scoping filters in this setting. We should have at the very least a clear vision of what these filters look like writ large before committing to the listener functions, so we don't dev ourselves into a corner.