Dho/lockless rrdcached

This PR addresses issues with rrd writing performance being serialized in workloads with many threads contending for the rrd mutex by introducing the ability to talk to rrdcached locally over a TCP socket. Each thread gets its own socket and is able to submit its own updates without waiting for other threads. This may ameliorate the need for using the RRDCACHED_ADDRESS environment variable in environments already making use of it.