Updated Entropy functions to use common/crypto

Entropy functions used to define their own random number generator and
entropy sources, now they rely on the common/crypto libraries.

EntropyBytes() has been renamed to EntropyGeneration().

DRNGSeed() has been changed from operating on a block to operating on a
slice of Heartbeat updates. This makes it easier for the block to order
heartbeats before calling the function, and removes a bit of complexity
from the DRNGSeed() function.

entropy_test.go has been updated to reflect all of these changes.