Chronos fails with `IndexError` in NBC
When running NBC against Medalla I'm getting the following on or shortly after startup:

```
.../nim-beacon-chain/vendor/nim-testutils/testutils/moduletests.nim(21) beacon_node
.../nim-beacon-chain/beacon_chain/beacon_node.nim(1198) main
.../beacon_chain/beacon_node.nim(892) start
.../beacon_chain/beacon_node.nim(842) run
.../vendor/nim-chronos/chronos/asyncloop.nim(278) poll
.../vendor/nim-chronos/chronos/transports/stream.nim(1329) readStreamLoop
.../vendor/nimbus-build-system/vendor/Nim/lib/system/fatal.nim(49) sysFatal
Error: unhandled exception: index 4096 not in 0 .. 4095 [IndexError]
```

OS: `Darwin Kernel Version 19.4.0: Wed Mar  4 22:28:40 PST 2020; root:xnu-6153.101.6~15/RELEASE_X86_64`