Fix founds with SonarQube

Rename this constant name to match the regular expression '^[A-Z][A-Z0-9...]*(_[A-Z0-9]+)*$'.

    -- org.elasticsearch.plugins.changes.ChangesPlugin
    -- org.elasticsearch.plugins.changes.module.ChangesModule
    -- org.elasticsearch.plugins.changes.rest.action.ChangesAction

Collection.isEmpty() should be used to test for emptiness
    -- org.elasticsearch.plugins.changes.rest.action.ChangesAction

Left curly braces should be located at the end of lines of code
    -- org.elasticsearch.plugins.changes.util.ConcurrentCircularBuffer

Tabulation characters should not be used
    -- org.elasticsearch.plugins.changes.util.ConcurrentCircularBuffer
    -- org.elasticsearch.plugins.changes.beans.Change
    -- org.elasticsearch.plugins.changes.beans.IndexChangeWatcher
    -- org.elasticsearch.plugins.changes.beans.IndexChanges

Override equals, hashcode in IndexChanges