Update to use httpx

Update finch to take advantage of the newer apis afforded by httpx

- bump finch to 0.3.0-SNAPSHOT
- replace finagle-http with finagle-httpx
- bump finagle to 6.22.1-MONOCACHE

All the changes should just be api churn, moving to httpx types rather
than the raw netty types.

fixes #93 
Wasn't sure what the right thing to do here was: add or update. Ditto for the overloads below.