* remove one cache, add another
This cache removes the need for rewinding in most attestation validation
flow since the attestations come from one of two epochs and must be
targetting a viable block.
Additionally, it also removes all state caches which are less likely to
be used over-all - more metrics are needed to track the rewinding.
On risk is that when chains don't finalize, we'll have lots of epochrefs
in memory meaning lots of validator key databases, most being exactly
the same. This can be addressed in any number of ways. Some of the
memory usage is mitigated by the fact that we previously had lots of big
state caches and now we're keeping only keys instead.
* cleanups
* doc
* removed the BlockPool type and all of the proxy functions around it - passing the chain DAG and the quarantine explicitly where appropriately - they don't need to be bundled in a type
* fixed the build after the rebase
Jacek Sieka
Viktor Kirilov