Logging parallel worker draught

Hi hackers,

Following my previous mail about adding stats on parallelism[1], this 
patch introduces the log_parallel_worker_draught parameter, which 
controls whether a log message is produced when a backend attempts to 
spawn a parallel worker but fails due to insufficient worker slots. The 
shortage can stem from insufficent settings for max_worker_processes, 
max_parallel_worker or max_parallel_maintenance_workers. It could also 
be caused by another pool (max_logical_replication_workers) or an 
extention using bg worker slots. This new parameter can help database 
administrators and developers diagnose performance issues related to 
parallelism and optimize the configuration of the system accordingly.

Here is a test script:

```
psql << _EOF_

SET log_parallel_worker_draught TO on;

-- Index creation
DROP TABLE IF EXISTS test_pql;
CREATE TABLE test_pql(i int, j int);
INSERT INTO test_pql SELECT x,x FROM generate_series(1,1000000) as F(x);

SET max_parallel_workers TO 0;

CREATE INDEX ON test_pql(i);
REINDEX TABLE test_pql;

RESET max_parallel_workers;

-- VACUUM
CREATE INDEX ON test_pql(j);
CREATE INDEX ON test_pql(i,j);
ALTER TABLE test_pql SET (autovacuum_enabled = off);
DELETE FROM test_pql WHERE i BETWEEN 1000 AND 2000;

SET max_parallel_workers TO 1;

VACUUM (PARALLEL 2, VERBOSE) test_pql;

RESET max_parallel_workers;

-- SELECT
SET min_parallel_table_scan_size TO 0;
SET parallel_setup_cost TO 0;
SET max_parallel_workers TO 1;

EXPLAIN (ANALYZE) SELECT i, avg(j) FROM test_pql GROUP BY i;

_EOF_
```

Which produces the following logs:

```
LOG:  Parallel Worker draught during statement execution: workers 
spawned 0, requested 1
STATEMENT:  CREATE INDEX ON test_pql(i);

LOG:  Parallel Worker draught during statement execution: workers 
spawned 0, requested 1
STATEMENT:  REINDEX TABLE test_pql;

LOG:  Parallel Worker draught during statement execution: workers 
spawned 1, requested 2
CONTEXT:  while scanning relation "public.test_pql"
STATEMENT:  VACUUM (PARALLEL 2, VERBOSE) test_pql;

LOG:  Parallel Worker draught during statement execution: workers 
spawned 1, requested 2
STATEMENT:  EXPLAIN (ANALYZE) SELECT i, avg(j) FROM test_pql GROUP BY i;
```

[1] 
https://www.postgresql.org/message-id/d657df20-c4bf-63f6-e74c-cb85a81d0383@dalibo.com

-- 
Benoit Lobréau
Consultant
http://dalibo.com