add gray routing predicate support.

[caichengw]

add gray routing predicate support based on request params(path,query,cookie or header), use consistent hashing other than random selection.

[kimmking]

Why not use a percent value for gray, but use a specific range value now?

[caichengw]

Why not use a percent value for gray, but use a specific range value now? @kimmking use an int value pair is more intuitive than a decimal value. and sometimes we have to directly indicate a range value (with start value and end value) rather than a range must start from zero.