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
Spotted an issue with ParallelSort. Added more ignored tests while fixing
Expose a way to ask a lucene index to sample index selectivity

o Scan a sample of index entries and compute uniqueSampledValues / sampleSize
Add a new CountsKey for index entry counts
Maintain index update counts from index proxies

o OnlineIndexProxy will ensure that the index counts are updated
o Fix bug when generate LazyIndexUpdates to never construct 'overlapping updates'
o Cleanup code in LazyIndexUpdates
o Make sure we update index update counts during population
Add minor version to CountsStore such that we can flush safely on index flip (outside of log rotation)

o Ensure we compare CountsKeys correctly by type
o Fix reading of index keys
o Add flushIndexCounts() to IndexStoreView
Remove index counts when dropping an index
Make index stats part of SchemaRead and add indexNumberOfEntries

Rationale: For planning to be sane, indices shouldn't appear or disappear out of nowhere, hence we need to be tied to the schema read lock
Add IndexStatisticsTest
Pass on transaction id to index updaters
Pull out and refactor IndexCountVisitor from CountingIndexUpdater
Only apply index count updates for unprocessed transactions during recovery

o Rename updateXYZ to incrementXYZ CountsStore related methods
o Push transactionId down to IndexUpdaters (and couple them to process only updates for a single transaction)
Replace use of LongRegister with DoubleLongRegister in counts store
Add new type of IndexKey for storing sampling results
Refactored counts package

o CountsAcceptor is now called CountsAccessor and also has read methods
o CountsState => CountsRecordState
o IndexKey is now split into Index{Size|Sample}Key
o CountsTrackerState now uses explicit calls per key type