internal/app/state: use internal/lockedfile

This is a pretty solid implementation backed by robust tests, with a much cleaner interface.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/state/state.go

@@ -19,9 +19,6 @@ type Store interface {
 	// List queries the store and returns a list of identities known to the store.
 	// Note that some or all returned identities might not have any active apps.
 	List() (identities []int, err error)
-
-	// Close releases any resources held by Store.
-	Close() error
 }
 
 // Cursor provides access to the store of an identity.