Consistent stacked bar colors for generic data

[philrz]

Repro is with Zui commit 729f9ee. This was first spotted during review of #2785 but we chose not to fit it into that iteration.

Thanks to the Brimcap plugin, the stacked bar chart for a pool based on imported pcap data uses the same consistent colors it did in the past using Zeek _path. However, for non-pcap data (such as the sample.zng from the Zui repo) we see in the attached video how the representation of the same data counts in the bars can change based on the zoom level, e.g., yellow+purple for a pair of types becomes orange+blue after zooming.

Repro.mp4

When discussing this as a team, @mccanne suggested that we could perhaps craft a solution that leverages consistent hashing to automatically pick colors and have them consistently land with the same types. I did some web searches and quickly found that others have struggled with this same problem, e.g., Grafana dealing with it at grafana/grafana#3250. The tl;dr I see is that after several years of noodling on it they did come up with a solution using the string-hash library. Perhaps we can borrow some ideas from their experience.