MEDIUM: servers: Replace idle-timeout with pool-purge-delay.

Instead of the old "idle-timeout" mechanism, add a new option,
"pool-purge-delay", that sets the delay before purging idle connections.
Each time the delay happens, we destroy half of the idle connections.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index c3318a4..977d4bd 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -472,6 +472,7 @@
 	defproxy.defsrv.minconn = 0;
 	defproxy.defsrv.maxconn = 0;
 	defproxy.defsrv.max_idle_conns = -1;
+	defproxy.defsrv.pool_purge_delay = 1000;
 	defproxy.defsrv.slowstart = 0;
 	defproxy.defsrv.onerror = DEF_HANA_ONERR;
 	defproxy.defsrv.consecutive_errors_limit = DEF_HANA_ERRLIMIT;