LUCENE-5611: indexing optimizations, dont compute CRC for internal-use of RAMOutputStream, dont do heavy per-term stuff in skipper until we actually must buffer skipdata

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/lucene5611@1590858 13f79535-47bb-0310-9956-ffa450edef68