Improve performance reading from network.

data_thread reads XML 2^10-1 bytes at a time, which is silly and
inefficient. Instead, create a buffer that doubles in size as necessary
and has a more reasonable initial buffer size.

When we get gzip content, start with a buffer size larger than our
gzipped contents, since we will probably need that anyway.
Add check for __thread in configure.
Use proper AC_DEFINE
Add configuration for rrdcached address
Fix config structure stuff.
Add code to talk to rrdcached directly w/o locks.
Read rrdcached response to our command
Helps to htons(3) the port...
Couple socket-related fixes:

 * Don't set non-blocking before connect
 * Make sure we write the full command
 * Use the proper pollfd offset :thumbsdown:
Merge branch 'dho-performance'

Conflicts:
	gmetad/conf.c.in