To optimize parallelism config in config file

Is your enhancement request related to a problem? Please describe. in config file of pulsar io , user can define the parallelism parameter, but there is a problem, for example In Mongodb Source, if this param is set to be 100000, when run in local run mode,100000 threads generated, but apparently this is wrong, how about the config set to be 10000000000? maybe u can say, this is set by user ,but we can build pulsar io stronger as posssible to reduce errors because users do not know more details , and they are not expected to be expert of all Storages.

besides this, in Mongo Source ,the parallelism just shoud be 1, if above 1, the result will be wrong

so, based on the 2 reasons, the parallelism config should be decided finally by the Source Class , just like kafka connect because the author of the Source Class knows concretely how the source runs

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