Multiple pubsub providers

[Rinse12]

Pubsub providers are needed to publish content for web/mobile users who can't run an IPFS node. Users submit their content to a pubsub provider, and the provider will publish the content to the rest of the network using pubsub.

As of now, plebbit-js only uses a single pubsub provider, but we need to have multiple providers to have better assurances of data delivery.

The publish process for a web/mobile client should be as follows:

• Publish to all available pubsub providers (maximum 3)
• If all of them fail, retry with timeout of 0ms 100ms 1000ms
• If it's still failing, then emit an error and throw

The subscribe process: TBD

[Rinse12]

Publishing to multiple pubsub providers is implements. Although subscribing is done for provider only, we may need to revisit this later.