Fix deferred FK check batching introduced by commit b7b27eb41a5

Fix deferred FK check batching introduced by commit b7b27eb41a5

That commit introduced AfterTriggerIsActive() to detect whether
we are inside the after-trigger firing machinery, so that RI trigger
functions can take the batched fast path.  It was implemented using
query_depth >= 0, which correctly identified immediate trigger firing
but missed the deferred case where query_depth is -1 at COMMIT via
AfterTriggerFireDeferred().  This caused deferred FK checks to fall
back to the per-row fast path instead of the batched path.

The correct check is whether we are inside an after-trigger firing
loop specifically.  Introduce afterTriggerFiringDepth, a counter
incremented around the trigger-firing loops in AfterTriggerEndQuery,
AfterTriggerFireDeferred, and AfterTriggerSetState, and decremented
after FireAfterTriggerBatchCallbacks() returns.  AfterTriggerIsActive()
now returns afterTriggerFiringDepth > 0.

Reported-by: Chao Li <li.evan.chao@gmail.com>
Author: Chao Li <li.evan.chao@gmail.com>
Co-authored-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/C2133B47-79CD-40FF-B088-02D20D654806@gmail.com