Some examples fixes

[s-celles]

• 02_subscribe_simple.jl raised:

┌ Warning: Assignment to `nmessages` in soft scope is ambiguous because a global variable by the same name exists: `nmessages` will be treated as a new local. Disambiguate by using `local nmessages` to suppress this warning or `global nmessages` to assign to the existing global variable.
└ @ C:\Users\scell\essai_mosquitto.jl:29
ERROR: LoadError: UndefVarError: `nmessages` not defined

• 03_subscribe_onconnect.jl raised:

┌ Warning: Assignment to `mrcount` in soft scope is ambiguous because a global variable by the same name exists: `mrcount` will be treated as a new local. Disambiguate by using `local mrcount` to suppress this warning or `global mrcount` to assign to the existing global variable.
└ @ C:\Users\scell\essai_mosquitto.jl:51
Connection of client 7kFtMi7iIhVddxW successfull (return code MOSQ_ERR_SUCCESS), subscribing to test/#
ERROR: LoadError: UndefVarError: `mrcount` not defined

• 04_multiple_clients.jl raised

  ERROR: LoadError: UndefVarError: `client` not defined

• Avoid global variables in some examples

• Rename 07_loop_forever.jl to 07_subscribe_loop_forever.jl