raft: Integrate snapshots into the raft.Storage interface.

Compaction is now treated as an implementation detail of Storage
implementations; Node.Compact() and related functionality have been
removed. Ready.Snapshot is now used only for incoming snapshots.

A return value has been added to ApplyConfChange to allow applications
to track the node information that must be stored in the snapshot.

raftpb.Snapshot has been split into Snapshot and SnapshotMetadata, to
allow the full snapshot data to be read from disk only when needed.

raft.Storage has new methods Snapshot, ApplySnapshot, HardState, and
SetHardState. The Snapshot and HardState parameters have been removed
from RestartNode() and will now be loaded from Storage instead.
The only remaining difference between StartNode and RestartNode is that
the former bootstraps an initial list of Peers.