Major Description
Removal in a distributed system requires a way to distribute the filters to the various instances of the mailqueue. This was implemented by pushing filters in a dedicated pulsar topic.
Since each instance needs to read the topic from the start to compile all the filters, it is necessary to have unique names for an exclusive subscription (see https://pulsar.apache.org/docs/3.3.x/concepts-messaging/#subscriptions)
If the instances used a shared subscription they would not receive filters that have already been received by other members of the subscription.
The current implementation uses Durable subscriptions and the uniqueness of the name is generated by adding an UUID. Unfortunately this means that a server restart will create a new subscription and the previous one will remain forever.
I recently discovered that pulsar has a limit on the total number of durable subscriptions 
eventually preventing the creation of new ones.
The filters should be migrated to non durable subscriptions (see https://pulsar.apache.org/docs/3.3.x/concepts-messaging/#subscription-modes)
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