HA stability fixes.

 o Cluster now allows same instance to restart a join cycle, as long as it joins from the same ip+port
 o FileLock gives more debugging details
 o ProposerState now uses value_1 rather than message payload in one incorrectly implemented branch.
Improve error logging when sending HTTP error response.

Ensure that we include the underlying exception as a root cause so
that our server-side logging includes it. Without this fix we just get
a WebApplicationException and its stacktrace with a null message, the
original exception was being returned to the client but thrown away on
the server side.
Add descriptions to status code classifiers in manual
Merge pull request #1593 from jakewins/hability

HA stability fixes.
Merge pull request #1600 from benbc/2.0-improve-http-error-response

Improve error logging when sending HTTP error response.
Diagnostics for surprising spurious failure.
Update the manual with new shell configuration properties.
Updates around MERGE documentation
Update to include single node patterns and clarify pattern use
Make sure thread has started before we observe it.
Fixes quorum check in ElectionState

The quorum check was wrong, leading to instances proceeding to do elections
 even when there were not enough instances left in the cluster, showing
 symptoms of split brain. This adds some tests and a fix for the issue
Fixes and reenables ElectionStateTest.testElectionFromDemoteIsRejectedIfNoQuorum
More stable park detection in lock test.
Fixes slave deadlock problem.

 o Instances now only block access if they were already in a state
   where they allowed access in the first place. This way instances
   won't grab multiple locks on the database, which previously lead
   to the database blocking forever (locked several times, only opened
   once).
 o Refactored out quorum check into a utilty method and replaced incorrect
   majority checks in HighAvailabilityMemberStateMachine and MultiPaxosContext.
revised movie graph example into a mini-app; placeholders for two more examples
Reject patterns with properties in relationships for pattern expressions
Documentation updates around patterns
prompt unsupported web browsers to try using webadmin
Documentation fixes related to formatting, tooling, content and PDF output. Added TV shows modeling example.
Merge pull request #1610 from nawroth/2.0-docs

Documentation fixes