v0.3.0 PG 13 support

• Change default configuration, Support PostgreSQL 13 new metrics (pg_slru, pg_shmem, pg_query13,pg_backup, etc...)
• Add a series new REST API for health / recovery status check
• Add a dummy server with fake pg_up 0 metric which serve before PgExporter is initialized.
• Add sslmode=disable to URL if sslmode is not given
• fix typo and bugs