Filesystem / DB contention with multiple readers

[tskisner]

This issue is just for keeping track of an investigation into observed "slowdowns" when multiple processes call get_meta() / get_obs() on different wafers (i.e. different framefiles), both within a single book and from separate books. This is from within the LoadContext operator, so each process creates a context, does the operation (either get_meta or get_obs) and then closes the context.

Mostly this is just anecdotal so far. For example running a single process that loads 7 wafers in sequence from one observation takes about 60 seconds per wafer (perlmutter compute node, reading data from CFS) to call get_meta + get_obs. Running with 8 processes, each reading 7 wafers in sequence from different observations, seems to take considerably longer.

A more systematic test is needed. The changes in #845 should also be tested to see if they help.