p2p: prevent writes at shutdown time

eth, eth/downloader: separate concerns, clean up test suite

As of this commit, we no longer rely on the protocol handler to report
write errors in a timely fashion. When a write fails, shutdown is
initiated immediately and no new writes can start. This will also
prevent new writes from starting after Server.Stop has been called.

eth, core: interrupt the chain processing on stop

Removed chain manager's select/channel approach when checking for
interrupts. Now using an atomic int32 instead which checked for every
block processed.

core/vm: fixed strange output for trace logging & error reporting

Added an additional channel which is used to interupt the chain manager
when it's processing blocks.

Invalid named pipe name

ipc socket always uses default path

RPC refactoring