RFR: JDK-8049244: XML Signature performance issue caused by unbuffered signature data

[Sean Mullan]

Please review my fix for JDK-8049244:

     http://cr.openjdk.java.net/~mullan/webrevs/8049244/webrev.00/

This is a direct port of the corresponding fix from Apache Santuario. No 
regression test since this is a performance related fix. However, the 
customer that reported this to the Apache Santuario project has already 
tested the patch and confirmed that it fixes the issue.

Thanks,
Sean