Use isolated connection configurations in the output plugin

Modify to use isolated connection configurations for each plugin instance.
The previous implementation had an issue where incorrect connections were used
due to ActiveRecord's connection pooling mechanism when the output plugin
was used multiple times.

lib/fluent/plugin/out_sql.rb

@@ -206,7 +206,7 @@ def start
       end
 
       SQLOutput.const_set("BaseModel_#{rand(1 << 31)}", @base_model)
-      ActiveRecord::Base.establish_connection(config)
+      @base_model.establish_connection(config)
 
       # ignore tables if TableElement#init failed
       @tables.reject! do |te|
@@ -224,7 +224,7 @@ def formatted_to_msgpack_binary
     end
 
     def write(chunk)
-      ActiveRecord::Base.connection_pool.with_connection do
+      @base_model.connection_pool.with_connection do
 
         @tables.each { |table|
           tag = format_tag(chunk.metadata.tag)