feat: support unprepared statement errors. fix #11

If Cassandra returns an unprepared error, the DAO will re-prepare it
and try the query again. This fixes #11, and handles the case where
Cassandra would go down, and then up again while Kong is still running.

Other changes:

- docs: base_dao and the factory have been given a better documentation.
- refactor:
  - `kong_query` is now the prefered term for _queries elements.
  - `params` becomes `args_keys` and `values` `args` for consistance with the driver.
  - The `_execute` method was renamed to `_execute_kong_query` and broken into smaller chunks.
  - Batch statements now need to use the raw `_execute` method (the new one, not the one renamed to `_execute_kong_query`.
  - Cassandra errors now also have the error code.

kong/dao/cassandra/apis.lua

@@ -16,32 +16,32 @@ function Apis:new(properties)
   self._schema = SCHEMA
   self._queries = {
     insert = {
-      params = { "id", "name", "public_dns", "target_url", "created_at" },
+      args_keys = { "id", "name", "public_dns", "target_url", "created_at" },
       query = [[ INSERT INTO apis(id, name, public_dns, target_url, created_at)
                   VALUES(?, ?, ?, ?, ?); ]]
     },
     update = {
-      params = { "name", "public_dns", "target_url", "id" },
+      args_keys = { "name", "public_dns", "target_url", "id" },
       query = [[ UPDATE apis SET name = ?, public_dns = ?, target_url = ? WHERE id = ?; ]]
     },
     select = {
       query = [[ SELECT * FROM apis %s; ]]
     },
     select_one = {
-      params = { "id" },
+      args_keys = { "id" },
       query = [[ SELECT * FROM apis WHERE id = ?; ]]
     },
     delete = {
-      params = { "id" },
+      args_keys = { "id" },
       query = [[ DELETE FROM apis WHERE id = ?; ]]
     },
     __unique = {
       name = {
-        params = { "name" },
+        args_keys = { "name" },
         query = [[ SELECT id FROM apis WHERE name = ?; ]]
       },
       public_dns = {
-        params = { "public_dns" },
+        args_keys = { "public_dns" },
         query = [[ SELECT id FROM apis WHERE public_dns = ?; ]]
       }
     }