Prompt user to accept legalese when datadir doesn't exist

miner: smart mining

xeth: removed mutex lock

core: during chain reorg rewrite receipts and transactions

Work is now handled and carried over multiple sessions. Previously one
session only was assumed, potentially resulting in invalid (outdated)
work

* Larger work / result queue
* Full validation option

xeth: log signed tx hash

Added PutBlockReceipts; storing receipts by blocks. Eventually this will
require pruning during some cleanup cycle. During forks the receipts by
block are used to get the new canonical receipts and transactions.

This PR fixes #1473 by rewriting transactions and receipts from the point
of where the fork occured.

core: genesis preparation

Implemented the --genesis flag thru which we can set a custom genesis
block, including the official Ethereum genesis block.

core/vm: added str to op

accounts, crypto: fixed file naming for windows

miner, xeth: fire log event during mining. Fix return raw tx

eth/downloader: drop peer if advertised TD but won't delvier

eth: calculate the correct TD, only update if better

eth, ethdb: fix a data race during startup/shutdown

Core uint64 ts

added net.version

Adjust .mailmap