Use Rock::IoState::writeableAnchor_ to detect rock entries open for writing.

Use Rock::IoState::writeableAnchor_ to detect rock entries open for writing.

Just e.mem_obj->swapout.sio presence is not reliable enough because we
may switch from writing to reading while the [writing] sio is still around.

More explicitly disabled on-disk collapsing of entries. The relevant code is
unstable under load [at least when combined with memory caching]. We were not
calling Ipc::StoreMap::startAppending() before so we probably did not fully
disk-collapsed entries before these temporary changes.

Added an XXX to mark an assert() that may fail if we allow on-disk collapsing.