Small promise chain hot path
Small promise chain hot path

Simple single-request handlers are by far the most common, so it's worth
optimizing for them. In local testing, this seems to improve throughput (as
measured by `ab -c 20 -n 10000
localhost:7231/en.wikipedia.org/v1/page/html/Foobar` from around 840 req/s to
about 870 req/s.

d00rman/restbase,wikimedia/hyperswitch,Pchelolo/restbase,cscott/restbase,inverno/restbase,d00rman/restbase,eevans/restbase,eevans/restbase,wikimedia/restbase,gwicke/restbase,milimetric/restbase,milimetric/restbase,wikimedia/restbase,gwicke/restbase,inverno/restbase,wikimedia/mediawiki-services-restbase,cscott/restbase,wikimedia/mediawiki-services-restbase,Pchelolo/restbase