[Realtime Anayltics] Support proper handling for the data collected from multiple time zones

[minudika]

Reporter : Inosh Goonewardena (inosh at wso2 dot com)
Jirra url : https://wso2.org/jira/browse/SIDDHI-16

Description :
CEP deployment might end up collecting data from multiple time zones. But at the moment CEP do the windowing based on local time zone server runs. This needs to be handled cleanly.

[suhothayan]

Need to add a doc on how this can be done using siddhi-execution-time extension

[suhothayan]

When the events have timezone field in them they can be converted to a unified time via the time:timestampInMilliseconds(), and processed in external time window with reorder:kslack(), or using a named aggregation.