store masterinfo data in elasticsearch

[yinzhigang]

Add a new masterinfo storage: elasticsearch

[siddontang]

@yinzhigang

Sorry for the late response, I am very busy these days.

IMO, now if we want to support multi stores to saving master info, we can create a sub-module master and move all related codes to it. like

master.go -> master/master.go
file_masterfile.go -> master/file.go
elasticserach_masterinfo.go -> master/es.go

[siddontang]

friendly ping @yinzhigang

[siddontang]

friendly ping again @yinzhigang