style(server): changed a LeaderInfo struct field from "startTime" to "StartTime"

Changed the LeaderInfo struct "start time" field from "startTime" to "StartTime" so that it is an exported identifier. This required adding the `json:"startTime"` structure field tag so that the encoding/json package correctly performs JSON encoding (i.e. the correct property name --> startTime).
Merge pull request #807 from Shopify/raft-server-stats-struct-field-tag-fix

style(server): changed a LeaderInfo struct field from "startTime" to "StartTime"