Increase default rpcthreads to 8

When solo mining, BFGMiner makes use of 3-4 long-lived connections: 1 for longpolling, 1 for submissions, and 1-2 for its own getblocktemplate calls. This barely fits with the default rpcthreads limit of 4, and means that when it has the total of 4 connections open, users cannot run any RPC calls of their own until one is closed by bitcoind timing it out. Ideally, we shouldn't block on idle connections, but while we do, increasing the default thread count to 8 seems reasonable IMO. Asked on #bitcoin-dev the other night and got no input, so I'm guessing nobody much cares.