investigate performance issue with updating the hierarchy

[propensive]

This is apparent, for example, when updating a layer using the new export menu. It appears like IPFS is called many times, possibly once for every layer in the tree.

This may be fixed by not using Layer.store so often, or by adding caching to Layer.store, but the latter may result in a memory leak if Layers are used as keys for a hash map.

[odisseus]

It turns out that the layers with their IPFS references are already cached in memory since PR #1173. However, the logs still show a lot of repeating messages "Checking for IPFS daemon".