bug: set "sticky" I/O format flags before forking threads

[devbww]

Any I/O manipulator that sets a per-stream I/O format flag
is not thread safe, as recent ThreadSanitizer warnings attest.
So, set std::ios::boolalpha in the benchmarks before we fork
any threads, rather than during concurrent output operations.

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This change is 

[codecov]

Codecov Report

Merging #1128 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #1128 +/- ## ========================================== - Coverage 93.49% 93.46% -0.04%  ========================================== Files 166 165 -1 Lines 12782 12779 -3 ========================================== - Hits 11951 11944 -7  - Misses 831 835 +4

Impacted Files	Coverage Δ
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	64.05% <100%> (+0.07%)	⬆️
...nner/benchmarks/single_row_throughput_benchmark.cc	93.1% <100%> (+0.77%)	⬆️
...ogle/cloud/spanner/benchmarks/benchmarks_config.cc	97.16% <100%> (ø)	⬆️
google/cloud/spanner/internal/spanner_stub.cc	67.03% <0%> (-2.2%)	⬇️
...loud/spanner/internal/partial_result_set_source.cc	93.24% <0%> (-1.36%)	⬇️
google/cloud/spanner/client.cc	89.18% <0%> (-0.91%)	⬇️
google/cloud/spanner/samples/samples.cc	86.08% <0%> (-0.29%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.55% <0%> (-0.16%)	⬇️
...anner/integration_tests/client_integration_test.cc	98.96% <0%> (-0.01%)	⬇️
google/cloud/spanner/keys.h	100% <0%> (ø)	⬆️
... and 4 more

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