Raft version 0.5.0

This is a major release and contains some very important backwards incompatible changes. This release supports dynamic membership changes!

Backwards incompatible changes:

• raft_entry_t now has a "type" field. If you zero your msg_entry_t struct then you could consider this a backwards compatible change.
• applylog now takes a raft_entry_t* instead of char* data, int size.
• All functions that take int node now take raft_node_t* node instead.
• raft_recv_entry() does not receive a node parameter anymore.

Changes

• Add appendentries message batching. Much faster replication!
• Add raft_add_non_voting_node
• raft_recv_appendentries accepts NULL node
• Add raft_apply_all
• Add raft_add_non_voting_node
• Add raft_remove_node
• Add node_has_sufficient_logs callback
• Add raft_node_is_voting
• Add raft_get_current_leader_node
• Add raft_get_node_from_idx
• Make raft_become_leader public
• Make raft_set_commit_idx public
• Make raft_get_commit_idx public

Bugfixes:

• A node voting for ID 0 doesn't break granting logic anymore
• Don't drop responses if the node's current_idx is 0
• AE response term is set to new current term determined from leader
• Prevent duped appendentries responses
• Election timeout doesn't promote single node to leader