Config option for buffer sizes in ring-jetty-adapter

In jetty, the request and response buffers can be configured by calling `Connector.set[Request|Response][Header|Buffer]Size()`

Attached patch exposes those as config options:
```
:buffer-size {:request  {:header 512 :body 1024}
              :response {:header 512 :body 1024}}
```
Each map (toplevel or :request/:response) can be set to a number instead, to set all sub keys, e.g.
```
:buffer-size 2048
;; or
:buffer-size {:request  1024
              :response 4096}
```
Being able to control buffer sizes is a prerequisite to test chunking behavior, see tests from #113