refactor!: use BackgroundThreads from common

[mr-salty]

fixes #1250

---

This change is 

[codecov]

Codecov Report

Merging #1262 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@ Coverage Diff @@ ## master #1262 +/- ## ========================================== - Coverage 94.87% 94.85% -0.02%  ========================================== Files 185 181 -4 Lines 13818 13771 -47 ========================================== - Hits 13110 13063 -47  Misses 708 708

Impacted Files	Coverage Δ
google/cloud/spanner/connection_options.h	100% <ø> (ø)	⬆️
google/cloud/spanner/connection_options.cc	91.89% <50%> (ø)	⬆️
...on_tests/rpc_failure_threshold_integration_test.cc	85.71% <0%> (-2.07%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	81.57% <0%> (-1.76%)	⬇️
google/cloud/spanner/client.cc	97.08% <0%> (-0.73%)	⬇️
google/cloud/spanner/value.h	92.8% <0%> (-0.11%)	⬇️
...anner/integration_tests/client_integration_test.cc	98.47% <0%> (-0.01%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.36% <0%> (ø)	⬆️
google/cloud/spanner/samples/samples.cc	88.04% <0%> (+0.47%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4b648a0...f8e8d8d. Read the comment docs.

[coryan]

Thanks! A couple of administrative changes:

• This is a breaking change and should be labeled as so (refactor!:) in the commit message.
• You also need to update the API baseline.

Reviewed 9 of 9 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @devbww and @devjgm)

[mr-salty]

Done (updated description and API dump).

[coryan]

Unfortunately @devjgm merged changes to the API dump, you need to rebase and so forth. It looks good though, ready to approve once the builds pass.

[coryan]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @devbww and @devjgm)