Enhance SPI functions of Sentinel

[cdfive]

Issue Description

Now Sentinel has some interfaces ,such as InitFunc,ProcessorSlot etc, which provide scalability for users. This is supported by SpiLoader class via default SPI mechanism of JDK.

We may refer to other frameworks like Dubbo and SOFARPC, enhance the SPI functions to support more scenarios, such as singleton/multiple instance, loaded by alias name etc, to make it better and convenient to use.