Support div_precision_increment sysvar in tidb

[yibin87]

Enhancement

https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_div_precision_increment

“This variable indicates the number of digits by which to increase the scale of the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.”

However in TiDB, this variable doesn't show the same behavior. In this FD, we shall make it compatible with MySQL, to satisfy the requirements of longer precision

[yibin87]

/assign @yibin87