Introduce cheaply clonable `ChunkStoreHandle` and `QueryCacheHandle`

[teh-cmc]

These are required for:

1. Making it easy to construct and pass QueryEngines around:

   // TODO(cmc): This needs to be a refcounted type that can be easily be passed around: the ref has
   // got to go. But for that we need to generally introduce `ChunkStoreHandle` and `QueryCacheHandle`
   // first, and this is not as straightforward as it seems.
   pub struct QueryEngine<'a> {
     pub store: &'a ChunkStore,
     pub cache: &'a Caches,
   }

2. Getting ingestion off of the main thread and parallelize it:

   • 4298

[teh-cmc]

@zehiko just hit that one as he was trying to integrate a QueryHandle/QueryEngine into an async Stream (which assumes multi-threaded environment, and therefore static lifetimes, etc).