Introduce debug capability for request content logging.

Currently undocumented, this introduces experimental support for logging
full request and response bodies. This is currently enabled by setting:

    org.neo4j.server.http.unsafe.content_log.enabled=true

In neo4j-server.properties, and by specifying the http log to output full
requests and responses:

    <pattern>%fullRequest\n\n%fullResponse</pattern>

As this uses mechanisms to fully duplicate incoming request and response
streams, it is strongly recommended to not run this in a production setting.
However, it should prove very useful for debugging and testing.
Merge pull request #3399 from chrisvest/2.1-req-content-logging

Introduce debug capability for request content logging.