Add hooks for session start and session end

Andrew Dunstan <andrew@dunslane.net>

Commit: cd8ce3a22c0b48d32ffe6543837ba3bb647ac2b2
Author: Andrew Dunstan <andrew@dunslane.net>
Date: 2017-11-15T15:16:34Z
Releases: 11.0
Add hooks for session start and session end

These hooks can be used in loadable modules. A simple test module is
included.

Discussion:  https://postgr.es/m/20170720204733.40f2b7eb.nagata@sraoss.co.jp

Fabrízio de Royes Mello  and Yugo Nagata
Reviewed by Michael Paquier and Aleksandr Parfenov

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