Reduce RPC overhead if read-only and/or change set is empty

For concurrent queries, Phoenix using Tephra is seeing a 20% overhead for transactional versus non transactional tables (see https://issues.apache.org/jira/browse/PHOENIX-1901). This is with no conflict detection occurring as the current tests are for immutable data in which this isn't required. One theory is that the extra overhead is due to the extra RPCs happening for canCommit and commit (versus non transactional tables). These RPCs can potentially be avoided if a connection is solely doing queries and/or the change set is empty.