Memory Leak

[Sam Bull]

There is a memory leak while syncing to the blockchain.

I think it's likely that the QEventLoop implementation from quamash is
causing the problem.

You can verify that the Qt app runs fine on its own by commenting out
lines bitcoin_network.py:80-82:

        self.spv = SPVClient(network, self.blockchain_view, bloom_filter,
                             self.merkle_block_index_queue)
        self.spv.on_network_chain_length_update(self._update_synced)

and bitcoin_app.py:43:

    wallets.local.WalletLocal.bitcoin_network.add_task()

Alternatively, to verify the blockchain downloading works without the Qt
loop, comment out bitcoin_app.py:31:

    asyncio.set_event_loop(loop)

and change the last line to:

    asyncio.get_event_loop().run_forever()

If you want some console output for blockchain progress, you can add
output somewhere around bitcoin_network.py:128 with something like:

        print(index, self.spv.network_chain_length)