automatic DAG pregeneration for smooth epoch transitions

Fix two additional download vulnerabilities

- backend: AutoDAG bool flag passed from cli/eth.Config to ethereum, autoDAG loop started if true
- backend: autoDAG loop start/stop, remove previous DAG
- cli: AutoDAG bool flag, off by default, but automatically ON if mining
- admin jsre: add startAutoDAG stopAutoDAG and makeDAG in miner section
- switch on/off DAG autogeneration when miner started/stopped on console

prefix dapp key/value entries in extradb

Tests updates

Cleanup

core/vm: optimisation on RETURN and updated tests

Storing tx receipts in extraDb

CLI, JSRE admin and  Solc improvements 

eth/downloader: fix test to it doesn't time out on a slow machine

eth: fix odd method names in peer set

multiple contract source for solidity compiler: returns contract array if multiple contracts. fixes #1023

Set the TEST_VM_COMPLEX env var to test complex vm tests which require a
lot of ram and quite some time.

Skipped tests due to large return value

core: block cache Has method thread safe