Intent to backport OAK-8071

[Michael Dürig]

Hi,

I intent to backport the changes we did for OAK-8071: 
http://svn.apache.org/viewvc?rev=1854515&view=rev

This adds some warning logging for specific cases where a commit is 
blocked for a long time (configurable via 
-Doak.segmentNodeStore.commitWaitWarnMillis) on a commit that is already 
in progress. Risk is low as this is pure logging. However it touches on 
some critical code paths.

Michael