Ensure contentLookup never has more than 5 active requests

[acolytec3]

I've noticed in the course of testing stuff on the live network that our contentLookup is queueing up more than 5 requests and gets stuck in this pending state where there are up to 20 open requests and none of them have resolved.

One example. Easily reproducible if you request the LightClientBootstrap on the beacon network with a local node.

We should try and reproduce this in a test locally and then find a fix.

The likely solution involves adding a timeout around in this section of code that will delete the request if the other node doesn't respond within a certain amount of time.