This commit adds pluggable consensus engines to go-ethereum. In short, it
introduces a generic consensus interface, and refactors the entire codebase to
use this interface.
The cache/dataset methods crashed with a nil pointer error if
cachesinmem/dagsinmem were zero. Fix it by skipping the eviction logic
if there are no caches/datasets.
Search always used the regular dataset size regardless of test mode. Fix
it by removing the redundant size parameter of hashimotoFull.
Fixes#3784
