After importing an used wallet, Receive shows already used addresses

**Expected behavior**

I'd expect Receive *always* gives an unused address.

**Actual behavior**

After importing a used wallet, Receive tab shows addresses that were already used.

**Steps to reproduce the problem**
1. Have a wallet where some external adresses from the start are already used (in my case, the balance was 0, not sure if it would work with adresses containing UTXOs)
2. Import the wallet to Jam
3. Click on Receive tab

**Specifications**
- Version: 0.3.0
- Platform: Start9
- Browser: Chrome

Reproduce the issue by importing a wallet with previously used external addresses, then opening the Receive tab. Trace how the Receive tab selects an address and verify that the result is unused after import, including the zero-balance case described here.

Written by the indexing model from the issue text.