A serial mode without using Pool

[axnsantana]

Including an option to run the scrapper in a serial mode without use Pool. The use of Pool is not available in environments like AWS Lambda (#114).

The mode selection uses the previous idea that if poolsize greater than 1, then run in parallel.

I am currently using this in a serverless application in AWS Lambda. The lambda function can be invoked multiple times to mitigate the loss of performance, simulating a parallel execution.

[lukaspistelak]

i have similar issue when run pool inside standard Thread...