Keep `aiohttp.ClientSession` active between calls to take advantage of the connection pool

[garciparedes]

This issue is intended to refactor the API Gateway's handling code (https://github.com/minos-framework/minos-api-gateway/blob/master/minos/api_gateway/rest/handler.py) such that it keeps an aiohttp.ClientSession between calls in order to take advantage of its connection pool and save time.

Here is the explanation included at the aiohttp's documentation: https://github.com/minos-framework/minos-api-gateway/blob/master/minos/api_gateway/rest/handler.py