Abbreviate common connection closed error.

When the client closes a connection to the server before recieving the full
reply, the server currently prints a lengthy stack trace to the log. This
stack trace is not helpful, because we already do the right thing here.

Replace stack trace with one-line log message noting the client closed the
connection.
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 #3377 from jakewins/2.1-connmsg

Abbreviate common connection closed error.
Merge pull request #3399 from chrisvest/2.1-req-content-logging

Introduce debug capability for request content logging.
Merge branch '2.1-maint' into 2.2-merge-2.1

Conflicts:
	community/server/src/main/java/org/neo4j/server/rest/transactional/ExecutionResultSerializer.java
	community/server/src/test/java/org/neo4j/server/rest/transactional/ExecutionResultSerializerTest.java
Merge pull request #3400 from chrisvest/2.2-merge-2.1

2.2 merge 2.1