rpc: Use UnixSocketServer from libjson-rpc-cpp

[chfast]

It builds, but it does not work. The "connection" is closed after a request, so you get something link this:

> eth.syncing
{
  currentBlock: 4418671,
  highestBlock: 4685520,
  startingBlock: 4418645
}
> eth.syncing
Error: write unix @->/home/chfast/.ethereum/geth.ipc: use of closed network connection
    at web3.js:3143:20
    at web3.js:6347:15
    at get (web3.js:6247:38)
    at <unknown>

> eth.syncing
{
  currentBlock: 4418680,
  highestBlock: 4685520,
  startingBlock: 4418645
}
> eth.syncing
Error: write unix @->/home/chfast/.ethereum/geth.ipc: use of closed network connection
    at web3.js:3143:20
    at web3.js:6347:15
    at get (web3.js:6247:38)
    at <unknown>

> eth.syncing
{
  currentBlock: 4420038,
  highestBlock: 4685524,
  startingBlock: 4418645
}