CouchbaseServerVersion should use clusterCompatibility version

CouchbaseServerVersion() naively picks the first server version it encounters, however in a mid-upgrade scenario, nodes in the cluster have different versions, and the cluster as a whole is only upgraded when all nodes are running on the same later version.

https://github.com/couchbase/sync_gateway/blob/3e536e5f7e66ab893ff47a02673182c628292786/base/bucket_gocb.go#L2253-L2257

cluster_compat_version is a encoded cluster-wide version number which reflects the Couchbase version of the entire cluster. This version is already used by other services determine feature compatibility, and seems cheaper to decode the major/minor versions from it than parsing the string formatted version.

function encodeCompatVersion(major, minor) {

   if (major < 2) {

     return 1;

   }

   return major * 0x10000 + minor;

}

http://src.couchbase.org/source/xref/6.0.0/ns_server/priv/public/ui/app-new/admin/mn-admin-service.js#encodeCompatVersion

We can get this out of the REST API at /pools/default or at /pools/default/buckets/:bucketName: (see attachment)

Prior art: goxdcr uses it for feature detection seen here:
https://github.com/couchbase/goxdcr/blob/e57c61b95f7a436b105a7592e978fe7a66d1f49e/utils/utils.go#L1318-L1336

https://github.com/couchbase/goxdcr/blob/e57c61b95f7a436b105a7592e978fe7a66d1f49e/base/simple_utils.go#L432-L450