Improvements on Mix protocol

[myui]

Fix MixServer to forgot models not being used

• Add a timer task to check obsolete sessions
• The timer task iterates over MixServerHandler.groupMap to check obsolete entries
• Change the value of MixServerHandler to have a last-accessed time for each session group
• Add a cleaner's check interval option

  Logging throughput and/or monitoring metrics on JMX/Ganglia

• processed msg/sec
• bytes/sec

  Add a forgetting scheme to PartialResult#addWeight

  Remove SPoF on MixServer

• Use a consistent hashing router on client(?)

  Support another type of MixMessage

• Change MixMessage to an interface/abstract class
• GetModelMessage to get weights/covars of given features

  Integration with online/streaming learners

• Use mix servers to mix training models among online/batch learners

  Make MixServer to a YARN application

[myui]

Supporting idle session sweeping within the commit [dcd14f1]