Transports should have dispose function

[enisdenjo]

Some transports have long-living connections, like mysql and neo4j. Currently we depend on the pubsub destroy event to close the connections, but this doesn't work because:

• Not clear to the user
• Prone to errors because user might just not provide it
• pubsub is used for subscriptions, not disposal

Instead of relying on the pubsub, each transport should implement a dispose function that closes any relevant connections during shutdown.