add authentication between the asimapd.py process and the asimapd_user.py process

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The main asimapd.py process has multiple connections (one for each authenticated IMAP client) to the asimapd_user.py process that handles all IMAP requests for an authenticated user. There is no authentication when talking to this child process.

A key is generated in the asimapd.py process for authenticating to each asimapd_user.py subprocess but it is not used.