Changed header validation method to function in preparation of
@karalabe's PR.

Removed full tx validation during state transitions

core: drop low gas tx

eth: add GasPriceOracle

p2p: prevent writes at shutdown time

Moved the queue to check to the checkQueue method so no undeeded loops
need to be initiated or sorting needs to happen twice.

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.

Transactions in the queue are now capped to a maximum of 200
transactions. This number is completely arbitrary.