Support using B/K/KB/M/MB/G/GB/T/TB/P/PB as unit in session variable exec_mem_limit

[yangzhg]

The variable exec_mem_limit in the current code only supports one byte as a unit. When the value is large, it is easy to make mistakes. Even using statements such as 110241024 is still not very convenient, so you need to support the use of B/K/KB/M/ MB/G/GB/T/TB/P/PB as the unit setting method
For example set exec_mem_limit=2G