core: increase the size limit of header RLP for chain accessor

**Problem**

Chain skeleton sync (batched headers sync) is impossible with
the current block size. Once the new stale peer is trying to sync its
headers, the following errors occurs during skeleton filling (some useless
logs are omitted):
```
DEBUG[02-20|14:00:46.499] Fetching batch of headers                id=8205ce1a7d69b885 conn=staticdial      count=1  fromnum=4       skip=0  reverse=false
DEBUG[02-20|14:00:46.908] Fetching batch of headers                id=8205ce1a7d69b885 conn=staticdial      count=1  fromnum=2       skip=0  reverse=false
DEBUG[02-20|14:00:47.153] Fetching batch of headers                id=8205ce1a7d69b885 conn=staticdial      count=1  fromnum=1       skip=0  reverse=false
DEBUG[02-20|14:00:47.426] Found common ancestor                    peer=8205ce1a number=0       hash=000000..000000
DEBUG[02-20|14:00:47.426] Directing header downloads               peer=8205ce1a origin=1
TRACE[02-20|14:00:47.427] Fetching skeleton headers                peer=8205ce1a count=192 from=1
DEBUG[02-20|14:00:47.427] Fetching batch of headers                id=8205ce1a7d69b885 conn=staticdial      count=128 fromnum=192     skip=191 reverse=false
DEBUG[02-20|14:00:47.427] Downloading receipts                     origin=1
DEBUG[02-20|14:00:47.427] Downloading block bodies                 origin=1
DEBUG[02-20|14:00:48.255] Filling up skeleton                      from=1
TRACE[02-20|14:00:48.256] Requesting new batch of headers          peer=8205ce1a from=1
DEBUG[02-20|14:00:48.256] Fetching batch of headers                id=8205ce1a7d69b885 conn=staticdial      count=192 fromnum=1       skip=0   reverse=false
...
TRACE[02-20|14:00:48.657] Skeleton filling not accepted            peer=8205ce1a7d69b885 from=1
DEBUG[02-20|14:00:48.657] Failed to deliver retrieved headers      peer=8205ce1a         err="delivery not accepted"
TRACE[02-20|14:00:48.657] Requesting new batch of headers          peer=8205ce1a         from=193
DEBUG[02-20|14:00:48.657] Fetching batch of headers                id=8205ce1a7d69b885 conn=staticdial      count=192 fromnum=193     skip=0   reverse=false
...
```

So there's a "delivery not accepted" error on attempt to build the skeleton
from received batch of received headers. Extended logs give us that every batch
of blocks contain only one header (the last from requested batch):
```
TRACE[02-20|14:00:48.656] Delivering headers                       accepted=false len(headers)=1 headers[0].Hash=5bb77b..06997d headers[0].Number=192 MaxHeaderFetch=192
```
So based on this information it's impossible to build the skeleton

**Explaination**

The problem is traced down to the node's peer. During headers retrieval
peer fetches the last block of the batch from its cache and tries to
retrieve the rest of them from ancients. Given the old maximum RLP header
size constraint of 700 bytes per header, the peer can't retrieve the whole
set of requested headers (MaxHeaderFetch, 192 headers each at least of
1006 bytes). Since the number of retrieved headers doesn't match the
requested one, chain accessor returns only the first header that was
retrieved from cache:
```
	// read remaining from ancients
	max := count * 700
	data, err := db.AncientRange(ChainFreezerHeaderTable, i+1-count, count, max)
	if err == nil && uint64(len(data)) == count {
		// the data is on the order [h, h+1, .., n] -- reordering needed
		for i := range data {
			rlpHeaders = append(rlpHeaders, data[len(data)-1-i])
		}
	}
	return rlpHeaders
```

**Solution**

Since `ReadHeaderRange` function specifies that the caller should limit the
number of requested headers to prevent DoS attack, it's safe to remove the maximum
bytes constraint.

**Additional notes**

After the fix I found that almost the same code change is present in the original
Geth commit ported to NeoX node implementation in #130:
447945e.
And thus, the problem should not be reproducible on the post-sync NeoX node.

Signed-off-by: Anna Shaleva <[email protected]>

Author: AnnaShaleva

core/rawdb/accessors_chain.go

@@ -334,8 +334,7 @@ func ReadHeaderRange(db ethdb.Reader, number uint64, count uint64) []rlp.RawValu
 		return rlpHeaders
 	}
 	// read remaining from ancients
-	max := count * 700
-	data, err := db.AncientRange(ChainFreezerHeaderTable, i+1-count, count, max)
+	data, err := db.AncientRange(ChainFreezerHeaderTable, i+1-count, count, 0)
 	if err == nil && uint64(len(data)) == count {
 		// the data is on the order [h, h+1, .., n] -- reordering needed
 		for i := range data {