MINOR: anon: store the anonymizing key in the CLI's appctx

MINOR: anon: store the anonymizing key in the CLI's appctx

In order to allow users to dump internal states using a specific key
without changing the global one, we're introducing a key in the CLI's
appctx. This key is preloaded from the global one when "set anon on"
is used (and if none exists, a random one is assigned). And the key
can optionally be assigned manually for the whole CLI session.

A "show anon" command was also added to show the anon state, and the
current key if the users has sufficient permissions. In addition, a
"debug dev hash" command was added to test the feature.