Add idle_session_timeout.

Add idle_session_timeout.

This GUC variable works much like idle_in_transaction_session_timeout,
in that it kills sessions that have waited too long for a new client
query.  But it applies when we're not in a transaction, rather than
when we are.

Li Japin, reviewed by David Johnston and Hayato Kuroda, some
fixes by me

Discussion: https://postgr.es/m/763A0689-F189-459E-946F-F0EC4458980B@hotmail.com