Hibernate subscription process after inactivity

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Add the :subscription_hibernate_after event store config setting used by subscription processes so they will automatically hibernate to save memory after a period of inactivity.

Hibernating a process will also force a garbage collection of the process' memory. As soon as a new message is received in the process mailbox the process will be resumed. This should have minimal impact on latency but can help to reduce memory leaks caused by large binaries.

By default subscription processes will be hibernated after 15 seconds of inactivity. This should provide a reasonable trade-off for minimising memory usage without affecting latency when the processes are busy. Setting the configuration value to :infinity will disable this behaviour.

Example configuration

# config/config.exs
config :my_app, MyApp.EventStore,
  serializer: EventStore.JsonSerializer,
  username: "postgres",
  password: "postgres",
  database: "eventstore",
  hostname: "localhost",
  subscription_hibernate_after: :timer.seconds(15)