client: don't populate Client::barriers from read accesses

If the entry doesn't exist, no need to create it.

Signed-off-by: Sage Weil <sage@inktank.com>
client/barrier: drop unused active_commit_interval

Signed-off-by: Sage Weil <sage@inktank.com>
client: fix barrier interval

(start, end) not (start, length)

Signed-off-by: Sage Weil <sage@inktank.com>
client: disable barrier support

The boost interval_set class is not available on centos6/rhel6.  Until that
dependency is sorted out, fix the build.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1254 from ceph/wip-barrier

libcephfs: disable barriers for now