fix: try to make test run a bit faster, and update some logs.

[yirutang]

Make custom executor no longer pass down to BigQueryWriteClient. The user have a choice of setting a custom BigQueryWriteClient nowadays. The custom executor will only be used in the scope of StreamWriter. This will reduce one test run time by 1 minute.

[codecov]

Codecov Report

Merging #279 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #279      +/-   ##
============================================
- Coverage     74.35%   74.33%   -0.03%     
  Complexity      382      382              
============================================
  Files            45       45              
  Lines          2566     2564       -2     
  Branches        101      101              
============================================
- Hits           1908     1906       -2     
  Misses          604      604              
  Partials         54       54              

Impacted Files	Coverage Δ	Complexity Δ
.../cloud/bigquery/storage/v1alpha2/StreamWriter.java	82.42% <100.00%> (-0.09%)	34.00 <0.00> (ø)

---

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 90ea84a...1c84612. Read the comment docs.