Run merging of layers in parallel

[goldmann]

If it is possible to run merging of layers in parallel - we should consider this.

[goldmann]

After skimming the code - it may be harder to do than I thought. While merging layers we do check for each layer we want to merge if the squashed layer already contains specific files. This means that we would need to synchronize on this call. But even if we synchronize, some other layer could add the same file in the meantime. It seems that parallel merging is not an easy improvement and it's even unclear if this is doable.