WIP: Libevm upstream sync with pivot change

[alarso16]

There's a few things that I should mention that are mostly related to structure.

• The downloader.go logic is very similar to that of the downloader.go in geth. The SnapSync() method essentially processSnapSyncContent. I removed a lot of the (seemingly) excessive logic here, but it seems to successfully move the pivot with rather rare errors.
• Prior to this PR, the state trie sync and atomic trie sync could be done parallelly or in either order. Now, it is changed to completely finish the atomic trie sync before beginning the snap sync, because we will perform atomic operations synchronously throughout the state sync. However, we could likely still do these parallelly. A channel was added to notify when atomic sync is completely, and we can thus start executing atomic operations as normal.
• We cannot postpone verify/accept operations on atomic operations until the pivot, because this can cause a deadlock with the P-Chain in avalancego. Because of this, we perform atomic operations synchronously, and then after the sync is complete, we perform the non-atomic operations in order as they are buffered to catch up the state trie to the atomic trie.
• Passing the lock from stateSyncerclient to Downloader is necessary because we need to block from queueing blocks once we finish the state sync, and then once the queue is cleared, we release the lock, allowing blocks to be verified as normal.
• The weird channels in Downloader weren't my idea - copied from geth with minimal revision. I'm not married to it, if you have a more readable suggestion.

My next steps:

1. There are likely some issues around exactly how atomic txs are handled during the final parts of the sync, and testing on mainnet (or testing multiple times on fuji)
2. WIP resuming a sync after canceled by SIGINT or something else.
3. The server must run in non-pruning mode for this to work, since the pivot can be too far back to respond to requests. A more permanent fix would be increasing the TipToBuffer and adjusting the pivot interval to find a "sweet spot".
4. None of the FUpgrade is here, so I need to rebase (probably very carefully) and retest to be able to work on fuji again.