etcdserver: autodetect v0.4 WALs and upgrade them to v0.5 automatically
fix import loop, add set to types, and fix comments
fix test import loop
add snapshotted integration test
Fix migration to allow snapshots to have the right IDs
Merge pull request #1743 from barakmich/auto_upgrade

etcdserver: autodetect v0.4 WALs and upgrade them to v0.5 automatically