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

Use the migration folder to upgrade WALs when autodetected.

/cc @philips @xiang90 @yichengq 
1. This sort of thing belongs in pkg/types
2. Suggestion - pull in the Set type from fleet and add a Contains method to it so we don't have a buttload of set-like helper functions all over the place