Content of PR:
Added VarPool:
Introduction: VarPool is similar to a global variable whose value can be dynamically modified. Variables are passed between different nodes in the task flow.
Difference: Global variables are constant attributes of the task flow, and currently there is no interface that can be dynamically modified.

Precautions:
Need to add the field var_pool in the t_ds_task_instance and t_ds_process_instance tables in the DB, the type is longtext.

How to use:(Currently only supports Python nodes)
Value: Just declare the variable name at LocalParams of the node.
Modification: ${setShareVar(variable name, value)} in the script.

Example:

Node a1:
	Script:
		value = 600 + 60
		${setShareVar(${shareVar1},value+6)}
		
Node a2:
	Custom parameters:
		shareVar1:"default"
	Script:
		print("shareVar1:",${shareVar1})
	operation result:
		"shareVar1:666"

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PR的内容：
新增VarPool：
作用：VarPool类似于一个值可动态修改的全局变量，在任务流下不同节点之间传递变量。
区别：全局变量为任务流的常量属性，目前没有接口可以动态修改。

注意事项：
需要在DB中的t_ds_task_instance和t_ds_process_instance表中增加字段var_pool，类型为longtext。

使用：（目前仅支持Python节点）
取值：只需在节点的LocalParams处申明变量名。
修改：在脚本中 ${setShareVar(变量名,值)}。

示例：

节点a1：
	脚本：
		value = 600 + 60
		${setShareVar(${shareVar1},value+6)}

节点a2：
	自定义参数：
		shareVar1:"default"
	脚本：
		print("shareVar1:",${shareVar1})
	运行结果：
		shareVar1:666

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Node a1:

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Node a2:

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result:

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Process: