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

Removed full tx validation during state transitions

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

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

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

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

Removed `Stop/Start` mechanism from the transaction pool.

This also reduces the time required spend in the VM