feat(connector): add rate limit in source parser

[kwannoel]

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What's changed and what's your intention?

Currently the rate limit logic is applied in the flow control executor which comes after source parser.

In order to reduce data in an epoch, the channel between source parser and flow control executor needs to be saturated, so it can trigger backpressure.

However, in certain cases where there is high source throughput, this channel can fill up with message chunks. So in the first epoch, if there's high amplification, there will be high barrier latency.

We shift the rate limit logic into the source parser for now as a quick workaround. But we will find a better place to place the rate limiting logic.

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[kwannoel]

Seems not required anymore, there should not be any channel between executors, only between different actors.