[C++] "make check-format" is slow

[asfimport]

We should probably run clang-format in parallel.

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #3011

_{Note: This issue was originally created as ARROW-3851. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 3011
#3011