* fix invalid state root being written to database
When rewinding state data, the wrong block reference would be used when
saving the state root - this would cause state loading to fail by
loading a different state than expected, preventing blocks to be
applied.
* refactor state loading and saving to consistently use and set
StateData block
* avoid rollback when state is missing from database (as opposed to
being partially overwritten and therefore in need of rollback)
* don't store state roots for empty slots - previously, these were used
as a cache to avoid recalculating them in state transition, but this has
been superceded by hash tree root caching
* don't attempt loading states / state roots for non-epoch slots, these
are not saved to the database
* simplify rewinder and clean up funcitions after caches have been
reworked
* fix chaindag logscope
* add database reload metric
* re-enable clearance epoch tests
* names
* Eliminate possibilities for range errors and overflows
* Handle more properly invalid requests for furute slots
* Eliminate the confusing surrounding the MAX_REQUEST_BLOCKS constant
Addresses https://github.com/status-im/nim-beacon-chain/issues/1366
* 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
