Release v2.5.6

• Support subquery for single measurement
• Support configuration file with yaml/toml format
• Fix issue #11 on Windows
• Support retention policy to query and fix db problem when query db.rp.measurement (#12)
  • create retention policy
  • alter retention policy
  • drop retention policy
• Fix gzip compress error
• Replace klauspost/pgzip with compress/gzip
• Forbid _internal database
• Update github.com/panjf2000/ants/v2 from v2.4.3 to v2.4.5
• Update github.com/json-iterator/go from v1.1.10 to v1.1.11