sdk/horizon: make transaction streaming stream only the escrow accounts

[leighmcculloch]

The transaction streaming code in the sdk/horizon package streams all transactions for a network. This was done to ensure that transactions were given to the sdk/state package in order to prevent unexpected things occurring like backwards state transitions or old balances.

The sdk/state package has been improved in the following issues and as a result should be able to handle receiving transactions out-of-order as long as for a single account they are received in order.

• sdk/state: dont let escrow account sequence number go backwards #312
• sdk/state: make safe to ingest txs out of order #319

The changes in those PRs also make duplicate transactions okay too.

We should revisit the sdk/horizon transaction streaming code in the horizon.Streamer and change it so that it only streams transactions from Horizon for the two escrow accounts.