[Moodle]: Adapt PHP-FPM settings

After a complaint about Moodle's performance (ticket #1602687), with people reporting to have timed out when trying to use it, we found out that PHP-FPM is reaching the maximum amount of child processes:

Sep 26 14:19:40 lga php-fpm[7265]: [NOTICE] [pool moodle] child 178895 started
Sep 26 14:20:01 lga php-fpm[7265]: [NOTICE] [pool moodle] child 178836 exited with code 0 after 810.405087 seconds from start
Sep 26 14:20:01 lga php-fpm[7265]: [NOTICE] [pool moodle] child 178897 started
Sep 26 14:39:49 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 8 children, there are 1 idle, and 14 total children
Sep 26 14:39:50 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 16 children, there are 1 idle, and 15 total children
Sep 26 14:40:59 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 8 children, there are 0 idle, and 13 total children
Sep 26 14:41:00 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 16 children, there are 1 idle, and 15 total children
Sep 26 14:42:00 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 8 children, there are 0 idle, and 11 total children
Sep 26 14:43:13 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 8 children, there are 0 idle, and 10 total children
Sep 26 14:49:42 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 8 children, there are 0 idle, and 14 total children
Sep 26 14:49:43 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 16 children, there are 0 idle, and 16 total children
Sep 26 14:49:44 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 32 children, there are 0 idle, and 18 total children
Sep 26 14:49:45 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 32 children, there are 0 idle, and 20 total children
Sep 26 14:49:46 lga php-fpm[7265]: [WARNING] [pool moodle] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers), spawning 32 children, there are 0 idle, and 22 total children
Sep 26 14:49:57 lga php-fpm[7265]: [WARNING] [pool moodle] server reached pm.max_children setting (32), consider raising it