statesync: improve e2e test outcomes (#6378)

I believe that this, in my testing seems to help the e2e state-sync
tests complete more reliably, by fixing some potential, range-related
slice building, as well as the way the test app hashes snapshots.

Additionally, and I'm not sure if we want to do this, but I added this
hook to the reactor that re-sends the request for snapshots during the
retry. This helps in tests prevent systems from getting stuck, but I
think in reality, it might create more traffic, and operators would
just restart a state-syncing node to get a similar effect.

config/config.go

@@ -834,6 +834,10 @@ func (cfg *StateSyncConfig) ValidateBasic() error {
 				return errors.New("found empty rpc-servers entry")
 			}
 		}
+		if cfg.DiscoveryTime != 0 && cfg.DiscoveryTime < 5*time.Second {
+			return errors.New("discovery time must be 0s or greater than five seconds")
+		}
+
 		if cfg.TrustPeriod <= 0 {
 			return errors.New("trusted-period is required")
 		}