Introduce timezone to time:currentTimestamp function

[minudika]

Purpose

Introduce timezone to time:currentTimestamp function

Goals

This allows providing timezone as an argument to time:currentTimestamp() function. Then it converts the current system time to the given time zone and returns in yyyy-MM-dd HH:mm:ss format.

Eg:

define stream InputStream (symbol string, price long, volume long);
from InputStream
select symbol , time:currentTimestamp() as currentTimestamp
insert into OutputStream;

This query returns, symbol from the 'InputStream' and the current timestamp of the system in yyyy-MM-dd HH:mm:ss format as currentTimestamp, to the 'OutputStream'.

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? yes
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes