miner: Add Work.stateMu to protect Work.state.

[benbjohnson]

This pull request should fix the panic caused by:

fatal error: concurrent map iteration and map write

goroutine 81 [running]:
runtime.throw(0xfb0690, 0x26)
	/usr/local/go/src/runtime/panic.go:616 +0x81 fp=0xc495b77408 sp=0xc495b773e8 pc=0x43f661
runtime.mapiternext(0xc495b77718)
	/usr/local/go/src/runtime/hashmap.go:747 +0x55c fp=0xc495b77498 sp=0xc495b77408 pc=0x41d31c
github.com/gochain-io/gochain/core/state.(*StateDB).Copy(0xc4ab406000, 0xc4204e9c00)
	/go/src/github.com/gochain-io/gochain/core/state/statedb.go:491 +0x2eb fp=0xc495b77788 sp=0xc495b77498 pc=0x7fbcdb
github.com/gochain-io/gochain/miner.(*worker).updateSnapshot(0xc420299340)
	/go/src/github.com/gochain-io/gochain/miner/worker.go:523 +0x106 fp=0xc495b777f8 sp=0xc495b77788 pc=0xa47176
github.com/gochain-io/gochain/miner.(*worker).commitNewWork(0xc420299340, 0x10d9e40, 0xc420038040)
	/go/src/github.com/gochain-io/gochain/miner/worker.go:489 +0x10c2 fp=0xc495b77d40 sp=0xc495b777f8 pc=0xa464b2
github.com/gochain-io/gochain/miner.(*worker).update(0xc420299340, 0x10d9e40, 0xc420038040)
	/go/src/github.com/gochain-io/gochain/miner/worker.go:266 +0x71c fp=0xc495b77fc8 sp=0xc495b77d40 pc=0xa4444c
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:2361 +0x1 fp=0xc495b77fd0 sp=0xc495b77fc8 pc=0x46ee21
created by github.com/gochain-io/gochain/miner.newWorker
	/go/src/github.com/gochain-io/gochain/miner/worker.go:156 +0x4d2

The issue was that after a Miner passes its Work to an Agent, it no longer has the ability to lock a mutex around it. The Miner mutates the Work.state by calling Commit() after the work is returned back to the miner.

Reported upstream: ethereum/go-ethereum#16449

[jmank88]

Should this one be a write lock?

[benbjohnson]

Good catch. Fixed in eb9e438.