Fix Cached graph load and update

// CachedGraphIndex: Should solve both (A) and (B)
// ren the immutable index dir to index_dirty
// any update goes directly to index_dirty
// write_update -> ren index_dirty to index 

// Continued
// cached a graph, encode the mutable index i.e, just copy-paste
// change the graph + index, this updated the mutable index
// write update -> this should not delete the existing graph; currently nukes existing index and copies over the temp one (A)

// Discontinued
// cached a graph, encode the mutable index i.e, just copy-paste
// decode the graph, index is loaded as immutable index 
// change the graph + index, this will copy the immutable index to mutable ver (B) and then updated the mutable index

Start by locating CachedGraphIndex and its graph load, mutable-index update, and write_update paths. Reproduce the cached-graph update cases described as (A) and (B), then verify that updates use index_dirty safely and do not delete the existing graph or index before the replacement is ready.