Fix bug of Value.toDate(string value) is not thread safe.

[brucewu-fly]

Fix bug of Value.toDate(string value) is not thread safe.

[brucewu-fly]

#108

[brucewu-fly]

The reasons why Date toDate(String value) is not thread safe are:

• The static member variables dateFormat, datePattern will be initialized many times when multiple threads invoke this method.
• The method parse() of SimpleDateFormat is not thread safe.

In general, there are three ways to solve this problem.

• Option1. Introduce synchronized. Date toDate(String value) -> synchronized Date toDate(String value)
• Option2. Use eager instantiation for dateFormat and datePattern. Put SimpleDateFormat[] dateFormat in ThreadLocal.
• Option3. Use eager instantiation for dateFormat and datePattern. Introduce third party libraries JODA time and use DateTimeFormat.forPattern to format time.

I prefer Option1 since it has minimal impact on the original code. In most cases we don't have multithreaded access so it won't cause thread context switch.

[shakeelmohamed]

The build failure here is very strange, I've sent an email to Travis CI support

[shakeelmohamed]

I've fixed some Travis CI configuration on the develop branch, merge it into your branch and the tests should run properly again