server: Correct mempool/CPU miner initialize order.
server: Correct mempool/CPU miner initialize order.

The CPU miner relies on the mempool, so the mempool has to be created
before calling the function to create the CPU miner.  When PR #568
introduced the mempool config struct, it moved the mempool creation
after the miner creation, which leads to the CPU miner crashing due to
trying to access a nil mempool.

This move the CPU miner creation after the mempool creation
appropriately.

qchain/btcd,qchain/btcd