cbbackupmgr DCP logging should be consistently formatted

With the addition of DCP V2 we should take the time to ensure that our DCP logging is consistent not only across versions but also in the same client.

All DCP logging should be in the form:
"(DCP) (<ud>bucket<ud>) (vb vbucket_id) error" or in Go style "(DCP) (%s) (%d) %s".