This is the first PR to retrieve neighbours in batches in traversals. This can drastically improve limited traversal runtimes on graphs with supernodes.

In the graph'sSingleServerProvider, this PR adapts how neighbours of a specific vertex are read in theexpand function. It now introduces a neighbour provider that is responsible for providing the neighbours in batches (which extracts a lot of code in theSingleServerProvider). For now we loop over all batches to get all neighbours inexpand. In a later PR (when the cluster case is also implemented),expand should return one batch per call.

The neighbour provider is set to a specific vertex and provides one batch of neighbours per call to itsnext function. Internally, it saves read neighbour batches to a cache. If the neighbour provider is set to a vertex for which the cache includes already all neighbours, the neighbour provider provides these cached batches instead of reading them again from memory.