Giraffe needs to better deal with systems where multiple processes opening the distance index causes slowdowns

[adamnovak]

Probably what we need to do is:

• At index build time, unmap, remap, and local-ify all the pointers, then mark the file read-only
• Have some kind of option to load the index as a stream instead of mapping it, and point users to it (or do it automatically?) if we think we still might have weird contention. Or make that the default and let a user ask to map it for tiny debugging runs.

When I run a single sample I get steady progress. When I run my samples in parallel (using a slurm array job) I get almost no progress and lots of watchdog errors. When I copy the 3 graph files to unique names for each sample for eahc array job I get steady progress again. 

Originally posted by @Andy-B-123 in #4020 (comment)