Increase download and upload buffers.

[dopiera]

This fixes #2657.

Experiments, which back these numbers are described in #2657.

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This change is 

[codecov]

Codecov Report

Merging #2945 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2945      +/-   ##
==========================================
- Coverage   90.56%   90.56%   -0.01%     
==========================================
  Files         298      298              
  Lines       20148    20172      +24     
==========================================
+ Hits        18248    18268      +20     
- Misses       1900     1904       +4

Impacted Files	Coverage Δ
google/cloud/storage/client_options.cc	92.59% <100%> (ø)	⬆️
...oogle/cloud/bigtable/internal/unary_client_utils.h	82.14% <0%> (-17.86%)	⬇️
google/cloud/storage/internal/complex_option.h	66.66% <0%> (-16.67%)	⬇️
google/cloud/storage/well_known_headers.h	70.37% <0%> (-3.71%)	⬇️
google/cloud/storage/well_known_parameters.h	68.88% <0%> (-3.34%)	⬇️
google/cloud/storage/internal/object_requests.h	97.01% <0%> (-2.99%)	⬇️
...le/cloud/bigtable/internal/async_retry_unary_rpc.h	64.15% <0%> (-1.24%)	⬇️
google/cloud/bigtable/internal/bulk_mutator.cc	90.21% <0%> (-1.09%)	⬇️
google/cloud/storage/internal/logging_client.cc	97.76% <0%> (-0.02%)	⬇️
google/cloud/bigtable/row_range.h	100% <0%> (ø)	⬆️
... and 9 more

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