mds: revert EMetaBlob::{fullbit,remotebit,nullbit} encoding optimization

Revert commit 40d56a97 (mds: optimize EMetaBlob::fullbit, remotebit,
nullbit encoding). This optimization creates small segments in the
result bufferlist of encoding EMetaBlob. Perf shows lots of CPU time
are used for allocating list node for bufferlist.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: deny reconnect for closed session

The client can have dirty caps or unsafe requests. Allowing it to
reconnect can compromise consistency.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>