CBGT should support multiple Couchbase URLs for bucket access + DCP

On sync gateway, we're in the process of adding support for multiple couchbase urls and we need CBGT to support multiple urls for bucket access and DCP 

Here is the Sync Gateway code where we initialize CBGT:

https://github.com/couchbaselabs/sync-gateway-accel/blob/65a20ec1985ff32535c04cd290a214a0a1ea5bd1/sg_accel/sg_accel.go#L152-L249

• cbgt.NewCfgCBEx() should take an array of URLs.
• Ditto for the "server" param in cbgt.NewManager()
• If any of the urls are pointing to servers that are not currently running, it should fallback to one of the other URLs.  
• Update the docs on the methods to reflect the changes, and include examples of what format is supported.  (http://, couchbase://)
• Add docs on the formats that are allowed for SSL connections (https://, couchbases://)
• Nice to have: the closer the format is to gocb / https://github.com/couchbaselabs/gocbconnstr, the less work we'll have to do on Sync Gateway to translate between the formats.  There are some docs on the gocb format here: https://developer.couchbase.com/documentation/server/current/sdk/go/managing-connections.html