add log message that warn user about running development version

Right now kea is logging which version is started:

DHCP4_STARTED Kea DHCPv4 server version 1.7.6-git started

We have clear release system:

if second number of version is odd - it's development
if second number of version is even - it's stable

Use simple modulo and if development version is detected - let's print out warning that running this version in production is "on your own responsibility"

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