MEDIUM: http_act: define set-timeout server/tunnel action

Add a new http-request action 'set-timeout [server/tunnel]'. This action
can be used to update the server or tunnel timeout of a stream. It takes
two parameters, the timeout name to update and the new timeout value.
This rule is only valid for a proxy with backend capabilities. The
timeout value cannot be null. A sample expression can also be used
instead of a plain value.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 81a90da..d4bfd1c 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -6265,6 +6265,24 @@
   the address family supports a port, otherwise it forces the source address to
   IPv4 "0.0.0.0" before rewriting the port.
 
+http-request set-timeout server|tunnel { <timeout> | <expr> }
+                                       [ { if | unless } <condition> ]
+
+  This action overrides the specified "server" or "tunnel" timeout for the
+  current stream only. The timeout can be specified in millisecond or with any
+  other unit if the number is suffixed by the unit as explained at the top of
+  this document. It is also possible to write an expression which must returns
+  a number interpreted as a timeout in millisecond.
+
+  Note that the server/tunnel timeouts are only relevant on the backend side
+  and thus this rule is only available for the proxies with backend
+  capabilities. Also the timeout value must be non-null to obtain the expected
+  results.
+
+  Example:
+    http-request set-timeout server 5s
+    http-request set-timeout hdr(host),map_int(host.lst)
+
 http-request set-tos <tos> [ { if | unless } <condition> ]
 
   This is used to set the TOS or DSCP field value of packets sent to the client