Use-after-free for server-side SSLContext with sni_callback

[gpshead]

I'm reserving an issue number to be used for a pending security fix (so that we have a GH-NNNNNN to refer to)

The title and description will be updated at the appropriate disclosure time.

Linked PRs

• gh-156293: Use-after-free for server-side SSLContext with sni_callback #158504
• [3.13] gh-156293: Use-after-free for server-side SSLContext with sni_callback #158507
• [3.15] gh-156293: Use-after-free for server-side SSLContext with sni_callback (GH-158504) #158514
• [3.14] gh-156293: Use-after-free for server-side SSLContext with sni_callback (GH-158504) #158515
• [3.12] gh-156293: Use-after-free for server-side SSLContext with sni_callback #158517
• [3.11] gh-156293: Use-after-free for server-side SSLContext with sni_callback (GH-158504) #158523
• [3.10] gh-156293: Use-after-free for server-side SSLContext with sni_callback (GH-158504) #158524

[gpshead]

see my gpshead fork's gh-156293-ssl-sni-callback-* branches for pre-prepared and tested backports for this to 3.10 through 3.15. @sethmlarson @hugovk