Changelog for libraft-0.2.1.0

Changelog for raft

Feature: Client requests are now cached by the current leader such that duplicate client requests are not serviced
Bug Fix: Fix issue in concurrent timer not resetting properly when the timeout was set to values under 500ms
Feature: Client read requests can now query entries by index or range of indices
Improvement: Raft nodes now only write to disk if the state changes during handling of events
Feature: Users can now specify what severity of log messages should be logged
Bug Fix: Receiving all data from a socket in the RaftSocketT monad transformer RaftRecvX typeclass instances inducing a deadlock has been rewritten
Feature: A PostgreSQL backend has been added for storage of log entries

Added a heartbeat broadcast by leader on read requests from client to ensure that the node is leader before responding to client
Each log entry is now linked by sha256 hash to the log entry immediately preceding it
Fixed bug where the last log entry was not being read from disk before starting the main event loop
Added quickcheck-state-machine tests for black-box testing of the raft-example client program