Move afterTriggerFiringDepth into AfterTriggersData

Move afterTriggerFiringDepth into AfterTriggersData

The static variable afterTriggerFiringDepth introduced by commit
5c54c3ed1b9 is logically part of the after-trigger state and in
hindsight should have been a field in AfterTriggersData alongside
query_depth and the other per-transaction after-trigger state.
Move it there as firing_depth.  Also update its comment to
accurately reflect its sole remaining purpose: signaling to
AfterTriggerIsActive() that after-trigger firing is active.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqFt4NGTNk7BinOsHHM48E9zGAa852vCfGoSe1bbL=JNFQ@mail.gmail.com