Introduce RPC to the framework of nginx channel

Add NGX_CMD_RPC command to nginx channel framework, so that workers
can request each other to execute self-defined RPC.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>
Indroduce lockfree stack into core library

With the help of lockfree stack, we can be easy to implement
lockfree algorithm when working on shared memory.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>
To share keepalive connections across all workers

In order to limit the number of connections for each upstream, it's
worth to share keepalive connections across all workers, so that idle
connections could be reused effectively by other workers.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>