Add firstScanDelay to Retriever.configStream

[indiketa]

Use case: We are using vertx-config to configure a database connection pool, we wanted to replace it with a new one when configuration is changed.

Retriever.configStream seems a perfect fit but it is not: Setting a scan period of 30 seconds causes an inacceptable delay when verticle starts (30 seconds) because the first configuration is received after the scan period.

In order to solve it: We are forced to Retriever.getConfig to get the configuration, and then open Retriever.configStream and discard the first read.

OR Set a lower scanPeriod that introduces unwanted overhead to the system.

Introducing a firstScanPeriod or similar into ConfigRetrieverOptions will be greatly appreciated.