DRY up signature request log.Fields

[g-k]

nit: Seeing this set of fields in a number of locations, I wonder if this should be moved to a struct that can output a log.Fields.

Originally posted by @ajvb in #782 (comment)

[g-k]

An alternative or additional option, #782 (comment)

The logrus readme includes this example:

  // A common pattern is to re-use fields between logging statements by re-using
  // the logrus.Entry returned from WithFields()
  contextLogger := log.WithFields(log.Fields{
    "common": "this is a common field",
    "other": "I also should be logged always",
  })

so that might be an option too. Unless you think it should be addressed in this PR I'll take a TODO and file an issue to clean that up.

[g-k]

This would dedupe some code and standardize the logs a bit more.