Add a Connection Pool

[tarekziade]

In case we want to control how many connections are open against the upstream server during a load test using tinap, we should add a connection pool in tinap based on the upstream host IP, and let the user configure how many sockets can be opened simultaneously against that server.

That will avoid draining FDs resources.

To be done only if we get that issue, since tinap is fully async and should be pretty performant when replaying raptor scripts

cc @acreskeyMoz

[acreskeyMoz]

@tarekziade Currently the socket connections are effectively 1:1 from firefox to the upstream server (say, WebPageReplay).

I'm concerned that the Connection Pool would change the performance characteristics of the app being tested. (For instance if we made it very small).