Add check for __thread in configure

Native compiler support for TLS is much faster than using pthread keys
with get/setspecific.
Configuration for talking directly to rrdcached

Wires up the configuration to be able to talk to rrdcached directly over
a TCP socket.
Add in support for talking directly to rrdcached

This removes the use of pthread mutexes in the case of updating an rrd.
Note that creating a new RRD still requires locking.
Read response from rrdcached

If we want to speak the protocol, we have to read the response. This
ignores errors that come back (if they do) because I don't have a good
idea of what to do with that information.
Need to htons(3) the port from the config.
Handle issues with sockets / nbio / polling better.
Merge pull request #129 from dhobsd/dho/lockless-rrdcached

Dho/lockless rrdcached