The cmds list may grow unbounded due to the POP3 analyzer being in
multiLine mode after seeing `AUTH` in a Redis connection, but never
a `.` terminator. This can easily be provoked by the Redis ping
command.
This adds two heuristics: 1) Forcefully process the oldest commands in
the cmds list and cap it at max_pending_commands. 2) Start raising
analyzer violations if the client has been using more than
max_unknown_client_commands commands (default 10).
Closes#3936
