State-only checkpoint state startup

Currently, we require genesis and a checkpoint block and state to start
from an arbitrary slot - this PR relaxes this requirement so that we can
start with a state alone.

The current trusted-node-sync algorithm works by first downloading
blocks until we find an epoch aligned non-empty slot, then downloads the
state via slot.

However, current
[proposals](ethereum/beacon-APIs#226) for
checkpointing prefer finalized state as
the main reference - this allows more simple access control and caching
on the server side - in particular, this should help checkpoint-syncing
from sources that have a fast `finalized` state download (like infura
and teku) but are slow when accessing state via slot.

Earlier versions of Nimbus will not be able to read databases created
without a checkpoint block and genesis. In most cases, backfilling makes
the database compatible except where genesis is also missing (custom
networks).

* backfill checkpoint block from libp2p instead of checkpoint source,
when doing trusted node sync
* allow starting the client without genesis / checkpoint block
* perform epoch start slot lookahead when loading tail state, so as to
deal with the case where the epoch start slot does not have a block
* replace `--blockId` with `--state-id` in TNS command line
* when replaying, also look at the parent of the last-known-block (even
if we don't have the parent block data, we can still replay from a
"parent" state) - in particular, this clears the way for implementing
state pruning
* deprecate `--finalized-checkpoint-block` option (no longer needed)

AllTests-mainnet.md

@@ -449,6 +449,11 @@ OK: 2/2 Fail: 0/2 Skip: 0/2
 + roundtrip                                                                                  OK
 ```
 OK: 2/2 Fail: 0/2 Skip: 0/2
+## Starting states
+```diff
++ Starting state without block                                                               OK
+```
+OK: 1/1 Fail: 0/1 Skip: 0/1
 ## Sync committee pool
 ```diff
 + Aggregating votes                                                                          OK
@@ -598,4 +603,4 @@ OK: 1/1 Fail: 0/1 Skip: 0/1
 OK: 9/9 Fail: 0/9 Skip: 0/9
 
 ---TOTAL---
-OK: 331/336 Fail: 0/336 Skip: 5/336
+OK: 332/337 Fail: 0/337 Skip: 5/337

beacon_chain/beacon_chain_db.nim

@@ -1101,6 +1101,21 @@ proc getState*(
     db.immutableValidators, db.statesNoVal[T.toFork], key.data, output,
     rollback)
 
+proc getState*(
+    db: BeaconChainDB, fork: BeaconStateFork, state_root: Eth2Digest,
+    state: var ForkedHashedBeaconState, rollback: RollbackProc): bool =
+  if state.kind != fork:
+    # Avoid temporary (!)
+    state = (ref ForkedHashedBeaconState)(kind: fork)[]
+
+  withState(state):
+    if not db.getState(state_root, forkyState.data, rollback):
+      return false
+
+    forkyState.root = state_root
+
+  true
+
 proc getStateRoot(db: BeaconChainDBV0,
                    root: Eth2Digest,
                    slot: Slot): Opt[Eth2Digest] =

beacon_chain/conf.nim

@@ -312,6 +312,7 @@ type
         name: "finalized-checkpoint-state" .}: Option[InputFile]
 
       finalizedCheckpointBlock* {.
+        hidden
         desc: "SSZ file specifying a recent finalized block"
         name: "finalized-checkpoint-block" .}: Option[InputFile]
 
@@ -763,11 +764,17 @@ type
         name: "trusted-node-url"
       .}: string
 
-      blockId* {.
-        desc: "Block id to sync to - this can be a block root, slot number, \"finalized\" or \"head\""
-        defaultValue: "finalized"
+      stateId* {.
+        desc: "State id to sync to - this can be \"finalized\", a slot number or state hash or \"head\""
+        defaultValue: "finalized",
+        name: "state-id"
       .}: string
 
+      blockId* {.
+        hidden
+        desc: "Block id to sync to - this can be a block root, slot number, \"finalized\" or \"head\" (deprecated)"
+      .}: Option[string]
+
       backfillBlocks* {.
         desc: "Backfill blocks directly from REST server instead of fetching via API"
         defaultValue: true