[YSQL] "Keyspace 'system_postgres' already exists" error when creating sequences concurrently

Jira Link: DB-4818 We currently store some sequence data (last val, is_called, etc) in a dedicated DocDB system table, that exists in the system_postgres namespace. That namespace is not pre-created, so "create sequence" calls will create it if needed by calling CreateNamespaceIfNotExists.

However, CreateNamespaceIfNotExists does not correctly handle the "If Not Exists" aspect for YSQL. Specifically, it checks if the namespace exists before, but if it gets an IsAlreadyPresent status that still gets reported as an error. See the code in: https://github.com/yugabyte/yugabyte-db/blob/master/src/yb/client/client.cc#L654 -- there the IsAlreadyPresent check at the end only handles YQL_DATABASE_CQL namespaces. That means that if many sequences are created concurrently at least some (possibly all-but-one) will fail.