injection_points: Fix incrementation of variable-numbered stats

injection_points: Fix incrementation of variable-numbered stats

The pending entry was not used when incrementing its data, directly
manipulating the shared memory pointer, without even locking it.  This
could mean losing statistics under concurrent activity.  The flush
callback was a no-op.

This code serves as a base template for extensions for the custom
cumulative statistics, so let's be clean and use a pending entry for the
incrementations, whose data is then flushed to the corresponding entry
in the shared hashtable when all the stats are reported, in its own
flush callback.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0v0U0yhPbY+bqChomkPbyUrRQ3rQXnZf_SB-svDiQOpgQ@mail.gmail.com
Backpatch-through: 18