Some bug fixed and improvement from OSDI experiments.
1. Add two more way to assign tiles, serpentine and static.
2. In order to support static assigne, we need extents can be sorted.
3. Add a configuration to config the maximum zeromq sockets.
   The zeromq default setting is too small for more than 256 workers.
4. The original count_nonzero can't count sparse correctly. Fixed this bug.