DB instance_start_time check is unnecessary

[Jens Alfke]

follow requires the JSON response from GET /db to contain an instance_start_time property, but it never uses the value of that property. Here's the check.

This makes follow incompatible with the Couchbase Sync Gateway (and Couchbase Lite when its listener/server plugin is used), because those implementations don't happen to include such a key in that response. Mostly because there is no explicit standard, and it hadn't seemed important enough to provide that information.

I can and will update our Couchbase codebases to return this key and make them compatible, but it also seems better for follow not to require a key that it doesn't otherwise use.