Fixed race condition: err "the pool is closed" during a rebalance

Commit from go-couchbase:
If Bucket.Do obtains a connection pool at the same time that another
goroutine refreshes the bucket's vbucket map, the bucket can end up with
an already-closed pool which will return errors trying to get a
connection.
Added a utility method Bucket.getConnectionToVBucket that does this
safely by retrying if it got a closed connection pool.
Added SlowServerCallWarningThreshold config variable

Set this to a value in milliseconds; Couchbase bucket requests that
take longer than this will cause a warning to be logged by go-couchbase.
Default value is 200ms.
Revert "CBLT-179:  set Version constants to tokens (instantiated by build process)"

This reverts commit 1b86eebeb7805213b566007197776362c4d768a0.