add health check call to libuv io plugin

There are some situations where an end user does not control the entire network and they want a way to check the health of connectivity.  This is sometimes also used to keep connections alive in environments where they'll be shut down.  This is particularly needed with PHP where we cannot run threads or any IO outside PHP's context, but it's needed elsewhere too.

Request here is to add a health_check() function that calls the underlying lcb function and delivers results to the application.  The intended usage would be for a developer, in some environments, to have their infrastructure monitoring to call this function on one or all app servers, which would then report back to the monitoring.  

See CCBC-801 for further info.