store: pass StoreUpdate as argument to ShardTries::apply_all

[mina86]

Rather than having ShardTries::apply_all create a new StoreUpdate and return it, change it such that it takes the update as an argument. This brings the method in line with other methods of the struct which also take reference to StoreUpdate as argument. It also renders some of the calls to StoreUpdate::merge unnecessary. The cost is that the call sites is now a bit more verbose since an explicit creation of the StoreUpdate is necessary.