Works include:
* define structures used by resource group in shared memory;
* insert/remove shared memory object when Create/Drop Resource Group;
* clean up and restore when Create/Drop Resource Group fails;
* implement concurrency slot acquire/release functionality;
* sleep when concurrency slot is not available, and wake up others when
releasing a concurrency slot if necessary;
* handle signals in resource group properly;

Signed-off-by Richard Guo <riguo@pivotal.io>
Signed-off-by Gang Xiong <gxiong@pivotal.io>