Switch params from a map[string]string to a []string{k1,v1,k2,v2,...}

With few keys < 10, this seems to perfom faster than a map.

The real gain is that the []string can be re-used. In 1000000 iterations, this
brought our allocation from 330.0MB to a constant of 14.5MB (and reduced the
amount of GCing by a measurable factor)