WL#9704: Defaults change, algorithm for calculating back_log

We have investigated the default value for back_log, and want to change this to a
value that is better suited for the MySQL server.

See:
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_back_log

In 5.7 the default value for back_log is based on the following formula, capped
to a limit of 900:

back_log = 50 + (max_connections / 5)

default value of max_connections=151

For MySQL 8.0 we will now change the formula to back_log=max_connections, while
preserving the default value to back_log=-1. Default value of '-1' implies that
the default value is determined at the time of initialization based on the value
of max_connections variable.

Keeping the default value of back_log to -1(autosized) will implicitly make it
equal to max_connections. MySQL will then be able to handle situations where all
connections are lost, and needs needs a re-connect. So a simple network down/up
which disconnects users, having the value of back_log equal to max_connections
will be just enough to smoothly reconnect. Based this logic the capping value of
900 will also be removed, so back_log will follow the value of max_connections
as default.

The default value of back_log should be calculated from the value of
'max_connections'. Default value will be capped to maximum limit permitted by
range of 'back_log' (65535).

    
   
back_log is read only variable. The value of back_log can not be changed
dynamically when server is running. Change algorithm for calculating default
value of back_log.
    
Old algorithm : back_log = 50 + (max_connections / 5)
New algorithm : back_log = max_connections
    
default value of `max_connections` is 151.
default value of back_log is -1 (autosized).
    
Old default value of back_log with default max_connections : 80 (50 + 151/5)
New default value of back_log with default max_connections : 151

Documentation link :
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_back_log