increase max concurrency to 50 (#203)

### Notes

Tested concurrency settings with a 2500 page PDF.  

Notes on various settings:

15 threads: 29 minutes
50 threads: 11 minutes
150 threads: 5 minutes


Memory usage:

15 threads:

Partition of a set of 530464 objects. Total size = 150187404 bytes.
(~150MB)

50 threads:
Partition of a set of 530606 objects. Total size = 150206646 bytes.
(~150MB)

150 threads:
Partition of a set of 530654 objects. Total size = 150221029 bytes.
(~150MB)

For now proposing we increase this to a max of 50. We can always keep
increasing later.

src/unstructured_client/_hooks/custom/split_pdf_hook.py

@@ -40,8 +40,8 @@
 
 DEFAULT_STARTING_PAGE_NUMBER = 1
 DEFAULT_ALLOW_FAILED = False
-DEFAULT_CONCURRENCY_LEVEL = 8
-MAX_CONCURRENCY_LEVEL = 15
+DEFAULT_CONCURRENCY_LEVEL = 10
+MAX_CONCURRENCY_LEVEL = 50
 MIN_PAGES_PER_SPLIT = 2
 MAX_PAGES_PER_SPLIT = 20