Require Redis version >= 3.2.0

Require Redis version >= 3.2.0

Since we now use Lua scripting for cleaning up c2s sessions
the minimum supported Redis version is 3.2.0 or above because
we need to work correctly with Redis replication mechanism.

****** BACKWARD INCOMPATIBILITY WARNING *******
** THIS SHOULD BE ADDED TO THE RELEASE NOTES **
*** PACKAGE MAINTAINERS SHOULD BE INFORMED  ***
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