perf(channelserver): move UserBinary and minidata to memory-only

UserBinary type1-5 and EnhancedMinidata are transient session state
resent by the client on every login. Persisting them to the DB on
every set was unnecessary I/O. Both are now served exclusively from
server-scoped in-memory maps (userBinaryParts, minidataParts).

Includes a schema migration to drop the now-unused type2/type3
columns from user_binary and minidata column from characters.

Ref #158

server/channelserver/session_lifecycle_integration_test.go

@@ -584,6 +584,7 @@ func createTestServerWithDB(t *testing.T, db *sqlx.DB) *Server {
 		sessions:        make(map[net.Conn]*Session),
 		stages:          make(map[string]*Stage),
 		userBinaryParts: make(map[userBinaryPartID][]byte),
+		minidataParts:   make(map[uint32][]byte),
 		semaphore:       make(map[string]*Semaphore),
 		erupeConfig:     _config.ErupeConfig,
 		isShuttingDown:  false,