Fix notifications lost during idle ping

[lukaszsamson]

This PR adds public Protocol.ping/2 and passes notify callback option to Protocol.new_status/2 when handling idle :timeout event.

Note: this new Protocol.ping/2 is not a callback of DbConnection behaviour. Downside is ping/1 called directly still defaults to [] options. Alternatives:

• extend the DbConnection callback surface
• keep using ping/1 but instead pass notify as state struct property

I'm happy to make the changes

Fixes #794

[josevalim]

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