[fly.cache] implement ttl function

[michaeldwan]

• expose the ttl function from the cache store to javascript
• implement the ttl function on the sqlite cache store
• redis returns -1 or -2 if a ttl isn't set or key not found. instead of passing this detail through to javascript, return an Option<i32> from the ttl function which is translated to a number if found or null if not found in js.
• write some tests