Compact in-memory epoch-history in logger

[hkimura]

Currently we keep every single history even though they are thousands of epochs earlier.
We don't need to keep all of them in-memory. We can discard 9/10 of them, for instance.
If a gleaner is lagging behind, it might have to scan more logs due to the history compaction, so we have to find a right balance between wasted memory and wasted work.

Or, write the history out to a file.

related code:

void LoggerRef::add_epoch_history(const EpochMarkerLogType& epoch_marker) {
...
if (control_block_->epoch_history_count_ >= LoggerControlBlock::kMaxEpochHistory) {
// TASK(Hideaki) To avoid this, we should maintain sparse history, like one history per
// tens of epochs. So far kMaxEpochHistory is enough big, so it won't happen.
LOG(FATAL) << "Exceeded kMaxEpochHistory. Unexpected.";
}