Fixes #4131

A slight refactor.
auto merge of #4132 : jtuchsen/servo/issue-4131, r=jdm

A slight refactor.
auto merge of #4000 : t29/servo/mime-sniffing, r=jdm

Issue: #3144

This PR addresses the second step of the ticket. i.e. move from a 1:1 sniffer:request task model to a shared sniffer task.