Signed-off-by: NKenan Yao <kyao@pivotal.io>

1) add one new type of gang: singleton reader gang.
2) change interface of allocateGang.
3) handling exceptions during gang creation: segment down and segment reset.
4) cleanup some dead code.

dispatcher/ directory

This commit has no logic change, it just contains movement of code across
files, to make dispatcher code clearer, and easier for unit testing.

Signed-off-by: Kenan Yao