OAK-1418: Read performance regression

Optimize the call path to ContentSessionImpl.checkLive() by turning
AbstractRoot to MutableRoot with a concrete reference to
ContentSessionImpl. This avoids some extra levels of indirection.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1573075 13f79535-47bb-0310-9956-ffa450edef68