[PATCH] pg_stat_statements: add last_execution_start column

Hi hackers,

This patch adds a `last_execution_start` column to `pg_stat_statements`, 
recording the start timestamp of the most recent execution of each 
tracked statement.

It supersedes the `stats_last_updated` series discussed here:
   
https://www.postgresql.org/message-id/flat/CAK7ymc+FxoVswo1ok_xDW-xPG-ZEZ8SAqCUkJ7WF04=0aQDvVQ@mail.gmail.com

The main criticism of that series was performance using 
`GetCurrentTimestamp()` inside the stats accumulation.  pgbench testing 
confirmed the concern of roughly 5–6% TPS regression on a 
short-transaction workload.

This patch takes a different approach.  Instead of calling 
`GetCurrentTimestamp()`, it uses `GetCurrentStatementStartTimestamp()`, 
which simply is a variable reading.
There is no syscall and no additional work in the hot path.

Benchmark (16-vCPU, pgbench -c8 -j4 -T60, explicit transactions with 15 
SELECT statements each):

   master HEAD:  ~4574 TPS (runs: 4636, 4585, 4500)
   patched:      ~4571 TPS (runs: 4577, 4560, 4575)
   difference:   ~0.1%

The column is initialized to the entry allocation time and updated on 
every call to `pgss_store()`.  It is reset by 
`pg_stat_statements_reset()` but preserved across minmax-only resets, 
consistent with `stats_since` semantics.

A monitoring query to find statements that have executed since the last 
observation could look like:

   SELECT query, calls, last_execution_start
   FROM pg_stat_statements
   WHERE last_execution_start >= $1  -- e.g. last check timestamp
   ORDER BY last_execution_start DESC;

Patch attached.

Best regards,
Pavlo Golub