Remove compiler warnings

src/couch-kvstore/couch-kvstore.cc:1389:15: warning: explicitly assigning a variable of type 'uint32_t'
      (aka 'unsigned int') to itself [-Wself-assign]
    itemFlags = itemFlags;

src/couch-kvstore/couch-kvstore.cc:1460:15: warning: explicitly assigning a variable of type 'uint32_t'
      (aka 'unsigned int') to itself [-Wself-assign]
    itemflags = itemflags;

src/couch-kvstore/couch-kvstore.h:249:12: warning: private field 'valuelen' is not used
      [-Wunused-private-field]
    size_t valuelen;

Change-Id: Iaec2830b82e2689f26d312c19bd1f4739d3d2b7a
Reviewed-on: http://review.couchbase.org/30668
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>