state/in-memory: Close() can deadlock with the cleanup goroutine

Close() calls wg.Wait() while holding the write lock. The cleanup goroutine started by Init() needs that same lock in doCleanExpiredItems(), so if the cleanup ticker fires between Close() acquiring the lock and the goroutine reaching Lock(), the two wait on each other and Close() never returns — a permanent hang with no timeout or error. Reproduces reliably with concurrent Close() calls; details and a fix in the linked PR.