MEDIUM: backend: add the "http-reuse aggressive" strategy

This strategy is less extreme than "always", it only dispatches first
requests to validated reused connections, and moves a connection from
the idle list to the safe list once it has seen a second request, thus
proving that it could be reused.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 9f7a6b5..08adccd 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -5042,6 +5042,12 @@
 			if (alertif_too_many_args_idx(0, 1, file, linenum, args, &err_code))
 				goto out;
 		}
+		else if (strcmp(args[1], "aggressive") == 0) {
+			curproxy->options &= ~PR_O_REUSE_MASK;
+			curproxy->options |= PR_O_REUSE_AGGR;
+			if (alertif_too_many_args_idx(0, 1, file, linenum, args, &err_code))
+				goto out;
+		}
 		else if (strcmp(args[1], "always") == 0) {
 			/* enable a graceful server shutdown on an HTTP 404 response */
 			curproxy->options &= ~PR_O_REUSE_MASK;
@@ -5050,7 +5056,7 @@
 				goto out;
 		}
 		else {
-			Alert("parsing [%s:%d] : '%s' only supports 'never', 'safe', 'always'.\n", file, linenum, args[0]);
+			Alert("parsing [%s:%d] : '%s' only supports 'never', 'safe', 'aggressive', 'always'.\n", file, linenum, args[0]);
 			err_code |= ERR_ALERT | ERR_FATAL;
 			goto out;
 		}