More subscriptions than permitted by PB_NATS_CLIENT_SUBSCRIPTION_POOL_SIZE

[film42]

https://github.com/mxenabled/protobuf-nats/blob/master/lib/protobuf/nats/client.rb#L18

Despite a firm limit on subscriptions at 512, we're seeing more than 2k connections.

Saw this opened here but I believe this bug is related to this gem: mperham/connection_pool#170

It's possible subscriptions are leaking out here: https://github.com/abrandoned/protobuf-nats/blob/84d632feb39684358b39727a4d5689c2545d3589/lib/protobuf/nats/client.rb#L245-L248

However, we should rewrite this implementation to use a single subscription like the official request method of the nats libraries.

[film42]

Spiked out an impl here: #2