Audit plugin "leaks" memory

Hello!

We see the audit plugin is causing growth of memory over time if threads remain connected for long. Memory IS released when thread disconnects, but many applications remain forever connected and the excess utilization becomes visible over the days.

Disabling audit plugins prevents memory growth. Here’s simple test case:

You can then monitor memory like:

Eventually the memory grows faster over time, as allocations appear to double at some point.

This resembles old bug for audit plugin:

And in performance schema we see that doubling in THD::Variables:

You can see initial growth takes some time, but again: over the days it just racks up gigabytes worth.