cubicweb #1818481 make twisted's threadpool size configurable [resolved]

Twisted's threadpool size defaults to 10, which means we virtually try to serve up to 10 simulateneous requests but connection-pool size defaults to 4 on CW-server side. That means that under heavy load, chances are high that some HTTP queries end up with a 500 error code / connection-pool exhausted.

done in3.13.1
load left0.000
closed by<not specified>