GH-41726: [C++][Parquet] Minor: moving EncodedStats by default rather than copying

[mapleFU]

Rationale for this change

Moving EncodedStats because it holds two std::string. This could benifit for non-SSO optimized data for FLBA/String statistics ( It seems to be useless for SSO optimized data?)

What changes are included in this PR?

1. construct EncodedStats by std::move
2. Making uncompressing size checking ahead of compressing

Are these changes tested?

Covered by existing.

Are there any user-facing changes?

No

• GitHub Issue: #41726

[github-actions[bot]]

:warning: GitHub issue #41726 has been automatically assigned in GitHub to PR creator.

[mapleFU]

@pitrou this is a short pr, would you mind take a fast glance?

[conbench-apache-arrow[bot]]

After merging your PR, Conbench analyzed the 7 benchmarking runs that have been run so far on merge-commit 8967ddc6d98584c88702c65763f33ec5f02324f7.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 10 possible false positives for unstable benchmarks that are known to sometimes produce them.