fix(max topic len): use the maximum length limit by default

src/emqx_topic.erl

@@ -42,7 +42,7 @@
 -type(word() :: '' | '+' | '#' | binary()).
 -type(words() :: list(word())).
 
--define(MAX_TOPIC_LEN, 4096).
+-define(MAX_TOPIC_LEN, 65535).
 
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 %% APIs