SDK adjustments

[ndzik]

• [x] Adapt subscribe() call to new backend implementation:
  • [x] subscribe("balanceProof", filterArgs)
  • [x] subscribe("txs")
  • [x] Also add a way to subscribe to both in a single call.
  • [x] BalanceProofs have to be adapted to new backend implementation, see erdstall-operator.
• [x] Add subscribe() to public Erdstall interface:
  • SDK would hang up if a user connects to Erdstall without subscribing, because it expects to receive TxReceipts.
  • Erdstall backend would need to pack TxReceipts in a Result call.
• [x] Add convenience constructors for LedgerConnection and EnclaveConnection:
  • Some users might not want to directly participate in Erdstall by issuing transactions, but just want to listen to events happening.
• [x] Move EnclaveConnection interface into erdstall.ts
• [x] Move the subscribe() interface method into some interface, this might be EnclaveWatcher or a new Subscriber.
• [x] Expose common namespace in the top-level index.ts, to ease importing from the SDK.

[RmbRT]

• [x] Client.erdstallConn: make into Promise/Future<LedgerConnection>

[ndzik]

All points were already addressed, are stale or already more advanced in their implementation than what this issue describes. Therefore closing it.