Add _finish_when_safe to work around DBD::Pg race condition

[TFBW]

Summary

Adds a private method _finish_when_safe in Mojo::Pg::Database, used by Mojo::Pg::Results instead of calling finish directly on DBI statement handles.

Motivation

There is a race condition in DBD::Pg which causes in-progress statement handles to be finished inappropriately when old statement handles are finished during an async DB query. This issue was previously concealed by a memory leak in Mojo::Promise which kept affected objects alive more or less indefinitely. Now that the leak is fixed, this DBD::Pg bug can cause async DB operations in close proximity to fail.

This change works around the problem by postponing finish operations if an async query is in progress. Most of the time there will be no such query in progress, and the handle is finished immediately. Where unsafe, a temporary array is created to hold any incoming statement handles, and the contents of this array (if any) are finished when the async operation completes.

References

• Problem report and analysis: PR #2268 for Mojo::Promise leads to regression with Mojo::Pg::Database mojo#2276
• Underlying DBD::Pg issue: Statement handle DESTROY slurps pending async queries bucardo/dbdpg#105
• Memory leak fix which exposed the issue: Fix memory leak in AWAIT_ON_READY mojo#2268

[kraih]

I've fixed the tests, please rebase on main.