* Add optional depth parameter to pubsub, and store in subscriptions
* Pass bank_forks into rpc_subscription; add method to check depth before notify and impl for account subscriptions
* Impl check-depth for signature subscriptions
* Impl check-depth for program subscriptions
* Plumb fork id through accounts
* Use fork id and root to prevent repeated account notifications; also s/Depth/Confirmations
* Write tests in terms of bank_forks
* Fixup accounts tests
* Add pubsub-confirmations tests
* Update pubsub documentation