Potential thread-safety fix for Memento from Luda.

(Partial?) fix for #177 - UKWA can help test this when @PsypherPunk is able to deploy this branch.

Email quoth:
> I attached [/waybackcore/src/main/java/org/archive/wayback/memento/] MementoUtils.java with corrected initialization of SimpleDateFormat.
> I think it should eliminate bug #177.  I used the same approach as can be find in openwayback commons (org.archive.util.ArchiveUtils) for thread safety.  
> There is also other possibility to use  Apache Commons Lang 3.x FastDateFormat, they claim that classes are thread safe and faster then SimpleDateFormat.  

>There is similar problem in formatting Link based format at cdx server module at org.archive.cdxserver.writer.MementoLinkWriter