Refactor env vars to clarify they are for individual ranks

[scott-routledge2]

Changes included in this PR

See here for more context. Essentially the environment variables for the buffer pool are split between cluster-level, node-level and rank-level granularity, which can cause confusion.

For example:
BODO_BUFFER_POOL_MEMORY_SIZE_MiB specifies the memory available to this rank for the buffer pool, whereas
BODO_BUFFER_POOL_STORAGE_CONFIG_1_SPACE_PER_DRIVE_GiB Is the space available for each node (assuming one drive).

Ideally IMO BODO_BUFFER_POOL_MEMORY_SIZE_MiB should just specify the amount of space available to all processes (on the same node) and then that space gets divided among ranks evenly, but then it creates a weird situation with BODO_BUFFER_POOL_MALLOC_FREE_TRIM_THRESHOLD_MiB still being per rank.

Adding "worker" makes it a bit more clear but then the environment variables too long e.g. BODO_BUFFER_POOL_WORKER_MALLOC_FREE_TRIM_THRESHOLD_MiB

Testing strategy

User facing changes

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All modified and coverable lines are covered by tests ✅

Project coverage is 66.42%. Comparing base (e4dcdf8) to head (9f6c537).
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Additional details and impacted files

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- Coverage   66.52%   66.42%   -0.11%     
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  Files         176      176              
  Lines       63332    63491     +159     
  Branches     8875     8891      +16     
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+ Hits        42133    42172      +39     
- Misses      18574    18691     +117     
- Partials     2625     2628       +3