Fix LongArrayBitVector to work with graphs over 8000 nodes

I only tested on a small bit vector and assumed longer ones would have the same structure, but they don't.

Java writes them in small chunks, separated by 5 bytes each, which makes it impossible to use with NumberMmap.

Instead, LongArrayBitVector will now load the whole file in memory like the Java class instead of mmapping. We should probably change this in the future, to either mmap it and work around the weird format, or change the on-disk format so LongArrayBitVector isn't needed anymore.