mds: use vector for context lists

mds: use vector for context lists

Commands executed:

    sed -i 's/\([^:]\)list<MDSInternalContextBase[[:space:]]*\*>/\1MDSInternalContextBase::vec/g' *.cc *.h
    sed -i 's/std::list<MDSInternalContextBase[[:space:]]*\*>/MDSInternalContextBase::vec/g' *.cc *.h

With various incidental fixes.

Fixes: http://tracker.ceph.com/issues/25111
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>