[hotfix][network] Reduce the synchronisation overhead in CreditBasedSequenceNumberingViewReader

[pnowojski]

Previously CreditBasedSequenceNumberingViewReader#isAvailable() was synchronising twice on the lock subpartitionView if there are no credits: first in subpartitionView.isAvailable() and second in subpartitionView.nextBufferIsEvent(). The first one subpartitionView.isAvailable() check is redundant (always true) if subpartitionView.nextBufferIsEvent() is also true so it should be safe to simplify the code.

Verifying this change

This change is already covered by existing network stack tests and most of the ITCases/e2e tests.

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[zhijiangW]

Thanks for this improvement @pnowojski , LGTM!