MINOR: compression: maximum compression rate limit

This patch adds input and output rate calcutation on the HTTP compresion
feature.

Compression can be limited with a maximum rate value in kilobytes per
second. The rate is set with the global 'maxcomprate' option. You can
change this value dynamicaly with 'set rate-limit http-compression
global' on the UNIX socket.
diff --git a/src/haproxy.c b/src/haproxy.c
index b322244..cfd6f84 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -113,6 +113,7 @@
 	.req_count = 0,
 	.logsrvs = LIST_HEAD_INIT(global.logsrvs),
 	.maxzlibmem = 0,
+	.comp_rate_lim = 0,
 	.unix_bind = {
 		 .ux = {
 			 .uid = -1,