redis pipeline for libvmod_redis

Thank you for picking up libvmod_redis from zephirworks... pulling together the redis connection leak and memory leak bits is helping me a lot.

Here's an implementation of redis_pipelines within the vmod. We use this feature heavily to cut down on the latency of many redis calls.

Most of vmod_pop is blatantly stolen from vmod_call and redis_common.

Please take a long look over what I've done here before accepting it... it works for me in minor testing but my C and varnish internals knowledge is not profound.