We want to be able to capture StateUpdates even if the Geth snapshot
trie is in a weird state and can't offer the snapshot we're looking
for. This adds our own implementation of the Snapshot() interface
so that we can continue collecting the necessary information to make
it available to the StateUpdates hook.
This makes several updates to support the blockupdates plugin.
I had to update several hooks that were using the wrong types, and
provide a way to get event.Feed objects into plugins without importing
event.Feed (which I did by having the plugin loader make them
available).
