This change introduces garbage collection for the light client. Historical
chain data is deleted periodically. If you want to disable the GC, use
the --light.nopruning flag.
* core: speed up GenerateChain
Use a mock implementation of ChainReader instead of creating
and destroying a BlockChain object for each generated block.
* eth/downloader: speed up tests by generating chain only once
This change reworks the downloader tests so they share a common test
blockchain instead of generating a chain in every test. The tests are
roughly twice as fast now.
