fix: avoid panic when Metadata.RefreshFrequency is 0

[napallday]

In #1578 , RefreshFrequency is used to initialize a ticker to periodically check whether to trigger a rebalance.
However, once it is set to 0, it would cause panic when calling pause := time.NewTicker(c.config.Metadata.RefreshFrequency), as shown in #1999.

When RefreshFrequency is 0, there is no backgroundMetadataUpdater to periodically refresh metadata. Thus, there is no need for loopCheckPartitionNumbers as well.

[napallday]

hi master @dnwe, I noticed one FVT failed and I found the error log was *** Test killed with quit: ran too long (16m0s).
I believe it's irrelevant to this commit. Could you help retry it? Thanks a lot!

[dnwe]

Thanks, good catch!