Introduce MAX_SUPPORTED_PRIORITY

Part of rabbitmq/rabbitmq-server#1590.

[#157380396]

Author: michaelklishin

include/rabbit.hrl

@@ -268,6 +268,9 @@
 
 -define(CHANNEL_OPERATION_TIMEOUT, rabbit_misc:get_channel_operation_timeout()).
 
+%% Max supported number of priorities for a priority queue.
+-define(MAX_SUPPORTED_PRIORITY, 255).
+
 %% Trying to send a term across a cluster larger than 2^31 bytes will
 %% cause the VM to exit with "Absurdly large distribution output data
 %% buffer". So we limit the max message size to 2^31 - 10^6 bytes (1MB