ZMQ: add options to configure outbound message high water mark, aka SNDHWM

doc/zmq.md

@@ -66,10 +66,21 @@ Currently, the following notifications are supported:
 The socket type is PUB and the address must be a valid ZeroMQ socket
 address. The same address can be used in more than one notification.
 
+The option to set the PUB socket's outbound message high water mark
+(SNDHWM) may be set individually for each notification:
+
+    -zmqpubhashtxhwm=n
+    -zmqpubhashblockhwm=n
+    -zmqpubrawblockhwm=n
+    -zmqpubrawtxhwm=n
+
+The high water mark value must be an integer greater than or equal to 0.
+
 For instance:
 
     $ bitcoind -zmqpubhashtx=tcp://127.0.0.1:28332 \
-               -zmqpubrawtx=ipc:///tmp/bitcoind.tx.raw
+               -zmqpubrawtx=ipc:///tmp/bitcoind.tx.raw \
+               -zmqpubhashtxhwm=10000
 
 Each PUB notification has a topic and body, where the header
 corresponds to the notification type. For instance, for the