Add option to enable compression per-server
Add option to enable compression per-server

Summary:
Before this diff, mcrouter would only load compression dictionaries if
compression was enabled system-wide or on a pool.
With this diff, mcrouter will load dictionaries even when compression is enabled
just for a single server.

Reviewed By: jmswen

Differential Revision: D3448994

fbshipit-source-id: 464124b50c4318d54da8d8cff2c7feff1bbe3c45

yqzhang/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,yqzhang/mcrouter,yqzhang/mcrouter,facebook/mcrouter,yqzhang/mcrouter