Use isolated connection configurations in the output plugin (#152)

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.

Signed-off-by: Haruki Hasegawa <h6a.h4i.0@gmail.com>

Author: h6ah4i

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)