# Functions
NewResourcePool 创建一个资源池子，capacity是池子中可用资源数量 maxCap代表最大资源数量 超过设定空闲时间的连接会被丢弃 资源池会根据传入的factory进行具体资源的初始化，比如建立与mysql的连接.
# Variables
ErrClosed is returned if ResourcePool is used when it's closed.
ErrTimeout is returned if a resource get times out.
# Structs
ResourcePool allows you to use a pool of resources.
# Interfaces
Resource defines the interface that every resource must provide.
# Type aliases
Factory is a function that can be used to create a resource.