Make Cursors instances of stream.Readable

[Ginden]

Streams are widely supported concept. There are many libraries to operate on streams, manage backpressure, map, filter results, pause them etc.

Moreover, since Node 10 stream.Readable automatically support asynchronous iteration protocol.

[btd]

@Ginden i am using node-pg-query-stream for this, it inhertis from Readable and based on cursor. But it does not work for node v12 (but PR exists)