Add lightwalletd full sync integration test

[teor2345]

Motivation

Most lightwalletd RPC methods need a cached state to operate on.

We can construct these integration tests (modulo sending txs) with pre-synced/indexed state, so we don’t have to worry about mainnet/testnet weather.

Tasks

• Attach a cached state disk to the lightwalletd integration tests
• Add a test harness that opens the cached state, but doesn't haven't have any configured seed peers
• Add a lightwalletd integration test for starts lightwalletd and waits for it to fully sync

Follow-Up Tasks

[teor2345]

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