Enhance parallel requests and bug fixes

[munrojm]

This PR expands the use of threadpools for parallel requests. It also fixes some pagination bugs with some user requested values of num_chunks and chunk_size.

[codecov-commenter]

Codecov Report

Merging #496 (a86db9d) into main (667848e) will decrease coverage by 0.58%.
The diff coverage is 97.50%.

@@            Coverage Diff             @@
##             main     #496      +/-   ##
==========================================
- Coverage   93.31%   92.72%   -0.59%     
==========================================
  Files          34       34              
  Lines        1271     1306      +35     
==========================================
+ Hits         1186     1211      +25     
- Misses         85       95      +10     

Impacted Files	Coverage Δ
src/mp_api/routes/robocrys.py	92.30% <ø> (ø)
src/mp_api/routes/synthesis.py	94.11% <ø> (ø)
src/mp_api/core/client.py	90.96% <97.46%> (+1.07%)	⬆️
src/mp_api/__init__.py	100.00% <100.00%> (ø)
src/mp_api/routes/materials.py	74.57% <0.00%> (-15.26%)	⬇️
src/mp_api/client.py	92.69% <0.00%> (-0.39%)	⬇️

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