cbft should check early that it's talking to a DCP-capable couchbase server version

From James Mauss usability test, he pointed cbft at a couchbase 2.5 cluster, and when he created an index, there were URP-related log error messages, but no web UI visible error messages.

cbft should instead check earlier, such at system startup time that the -server cmd-line param is to a recent-enough couchbase server version.

Start at the startup handling for the -server command-line parameter and trace how cbft connects to the Couchbase server. Determine how to detect whether the server supports DCP and how startup failures are surfaced; done means an unsupported server version is rejected early with a visible error instead of later URP log errors.

Written by the indexing model from the issue text.