Add support for request-received middleware

[niftylettuce]

The request-received middleware at https://github.com/cabinjs/request-received exposes a Symbol.for property, which this package could conditionally consume if interested (as opposed to just generating a new Date().

e.g.

node-connect-datadog/lib/index.js

Line 32 in cc04b2d

req._startTime = new Date();

would change from req._startTime = new Date(); to req._startTime = req[Symbol.for('request-received.startTime')] ? req[Symbol.for('request-received.startTime')] : new Date()

[rochdev]

What is the benefit of this approach? Wouldn't putting the Datadog middleware before the logger result in a more precise timing?