Modify EthashListener API

[zilm13]

Regarding #1084
We should finalize it while it's in rc and going to release soon. Best time for it.
Latest flow we've agreed with @mkalinin looks like this:
light dataset settings, dataset already generated and read from disk , events:

DATASET_PREPARE
DATASET_PREPARED

full dataset settings, dataset already generated and read from disk , events:

DATASET_PREPARE
DATASET_PREPARED

light dataset settings, dataset generated, events:

DATASET_PREPARE
LIGHT_DATASET_GENERATE_START
LIGHT_DATASET_GENERATED
DATASET_PREPARED

full dataset settings, dataset generated, events:

DATASET_PREPARE
FULL_DATASET_GENERATE_START
LIGHT_DATASET_GENERATE_START
LIGHT_DATASET_GENERATED
FULL_DATASET_GENERATED
DATASET_PREPARED

I think it covers most cases users may need to track, but I'd change one of DATASET_PREPARE and DATASET_PREPARED names to decrease number of mistakes in usage.