Pubsub subsystem stops working after some time

#### Version information:

> ipfs version --all
go-ipfs version: 0.4.18-
Repo version: 7
System version: amd64/linux
Golang version: go1.11.1

#### Type:

bug

#### Description:

We have a number of nodes communicating via pubsub. They are a mixture of 0.4.17 and 0.4.18. Sometimes a node goes into a state where pubsub stops working. On node `A` there is plenty of pubsub traffic on topic `topic`. On node B, which is a peer to `A`, pubsub on the same topic is silent.

`ipfs pubsub peers` is empty and remains empty even when trying `ipfs pubsub sub --discover `. The only thing that gets out of this state is to restart the ipfs daemon.