OAK-1184: Uploading large number of files to single folder fails.

Need to flush already when refcount reaches SEGMENT_REFERENCE_LIMIT,
not just when it exceeds it. Otherwise there's a conflict on the 0xff
refid used for the current segment.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1545696 13f79535-47bb-0310-9956-ffa450edef68