Implement subscription pooling in web3.js/connection

[steveluscher]

Problem

If two callers subscribe, for instance, using onAccountChange() for the exact same address, we set up two subscriptions.

Proposed Solution

Implement subscription pooling.

Create a map of people who care about a given subscription:

const accountChangeListeners: Map<
  string,  // Pubkey
  AccountChangeCallback[]
> = new Map();

When that list of callbacks goes from zero to one, set up the subscription. When it falls from 1 to zero, unsubscribe.

When a subscription publishes a payload, broadcast it to all of the callbacks.

[nilpferdschaefer]

I second this - was just looking at this issue and handling it locally at the moment! Subscription pooling would be great.

[steveluscher]

Nice! Any feedback on our implementation @LeFinc? #24473.