Ttfb should not be negative

[aarshkshah1992]

fixes #54 .

[guanzo]

lgtm

[guanzo]

@aarshkshah1992 still getting negative values somehow

2023-03-07T00:44:44.633Z	17488e86-27d8-4e66-b0f5-5e67bb8ce449	INFO	Validation err: ttfbMs is negative {
  cacheHit: false,
  url: 'https://13.208.246.7/ipfs/QmfEWNbcZQfYDgePRvxDDGH9WoC4td3E1mgNLbRpDEoUuf?format=raw',
  startTime: '2023-03-07T00:44:38.210681964Z',
  numBytesSent: 181,
  requestDurationSec: 0.706223358,
  requestId: 'f478be4edbcb6d4103feec09f9cc8d38',
  httpStatusCode: 502,
  httpProtocol: 'HTTP/1.1',
  ttfbMs: -1678149878210,
  range: '',
  referrer: '',
  userAgent: 'bifrost-gateway/2023-03-01-adb95e1',
  nodeId: '8f3fffe6-5056-47d8-9c5c-6d44ed0c4ff6',
  ifNetworkError: '',
  nodeIpAddress: '13.208.246.7',
  host: '13.208.246.7',
  pathPrefix: 'ipfs',
  cid: 'QmfEWNbcZQfYDgePRvxDDGH9WoC4td3E1mgNLbRpDEoUuf',
  filePath: '',
  clientId: null,
  requestByteOffset: 0
}