RFR: 8145934: Make ttyLocker equivalent for Unified Logging framework

[Marcus Larsson]

Hi,

Please review the following patch adding support for non-interleavable 
multi-line log messages in UL.

Summary:
This patch adds a LogMessage class that represents a multiline log 
message, buffering lines that belong to the same message. The class has 
a similar interface to the Log class, with printf-like methods for each 
log level. These methods will append the log message with additional 
lines. Once all filled in, the log message should be sent to the the 
appropriate log(s) using Log<>::write(). All lines in the LogMessage are 
written in a way that prevents interleaving by other messages. Lines are 
printed in the same order they were added to the message (regardless of 
level). Apart from the level, decorators will be identical for lines in 
the same LogMessage, and all lines will be decorated.

Webrev:
http://cr.openjdk.java.net/~mlarsson/8145934/webrev.00/

Issue:
https://bugs.openjdk.java.net/browse/JDK-8145934

Testing:
Included tests through JPRT

Thanks,
Marcus