couchbase Cfg doesn't implement CAS correctly

@nimishzynga found a concurrency race in the couchbase-bucket implementation of the Cfg (cfg_cb.go) where the CAS implementation doesn't work and is broken...

https://github.com/couchbaselabs/cbgt/blob/master/cfg_cb.go#L103

Solution ideas include not using a client-side cache (cfgMem), but always just going to the couchbase bucket.

Start with cfg_cb.go at line 103 and trace how Cfg performs CAS through the couchbase-bucket implementation, including the cfgMem client-side cache. Verify the behavior under concurrent updates against the Couchbase bucket; it is done when CAS operations no longer race and preserve the expected update semantics.

Written by the indexing model from the issue text.