Add ContentCache for each portal subprotocol (Portal wire protocol)
Probably each subnetwork would benefit from having a content cache. This means caching the most recently requested content items, even when they are not supposed to be stored by that node.

The first network to apply it on would be history network. There should be a flag to be able to disable/enable it.

Perhaps LRU (https://github.com/status-im/nim-eth/blob/master/eth/p2p/discoveryv5/lru.nim) can be used for it as cache.
