Set replication model to 's' when shard_replication_factor is 1

The `citus.replication_model` setting is confusing and unnecessary. Moreover, the default is 'statement' for historical reasons, but many features require 'streaming'.

We should deprecate the GUC and automatically set tables to repmodel 's' if the replication factor is 1. Existing tables with replication factor 1 should be updated to 's'.