core: use buffered ch for subscription

[ceyonur]

After the refactor the channel changed from buffered to unbuffered. The comment said Buffered to avoid locking up the event feed, if that's the case (it seems like can be) should the channel be kept buffered and even with the the buffer capacity >1?

[rjl493456442]

The buffer channel is unnecessary.

Once the event is emitted via the channel, it will be consumed very quickly.
In the code snippet below, the event will try to spin up a background indexer
if it was not active, and then update some internal flags. The whole procedure
is regarded very fast and very unlikely to block the feed.

In another word, if the event consumer is very slow, the feed will be blocked
anyway theoretically no matter what the size is.

		case head := <-headCh:
			if done == nil {
				stop = make(chan struct{})
				done = make(chan struct{})
				go indexer.run(rawdb.ReadTxIndexTail(indexer.db), head.Block.NumberU64(), stop, done)
			}
			lastHead = head.Block.NumberU64()