Fix Perl async callback refcounts

[xintenseapple]

Summary:

• Validate async Perl callbacks as CODE refs, hold an owned Perl reference while they are pending, and release it on dispatch, setup failure, or request cleanup.

Verification:

• git diff --check; ./auto/configure --with-http_perl_module --without-http_rewrite_module; Perl XS and ngx_http_perl_module.c compiled; final link fails in this environment because libperl is missing

Fixes #26