And will any say when my bell of quittance is heard in the gloom, (17)
Here, the speaker does away with any "if" statement – he knows that his "bell of quittance" will someday be heard. So he doesn't bother with "If my bell of quittance is heard." Death is unavoidable, so the bell marking his death is, too.
And a crossing breeze cuts a pause in its out-rollings, (18)
Death, and the accompanying funeral bells, might be unavoidable, but somehow nature is able to interrupt it. The "crossing breeze" is able to "cut a pause," or interrupt the sound of the bell. It's as though the breeze were able to disrupt the flow of time itself.
Till they rise again, as they were a new bell's boom (19)
Of course, the "breeze" can't stop the flow of time forever – the sound of the bells picks up again when the breeze dies down. But the words "rise again" suggest the possibility of life after death by alluding to the Christian idea of physical resurrection on Judgment Day, and the alliteration of the "new bell's boom" draws our attention to the "newness" of the bells, which suggest the possibility of renewal. At least the poem doesn't end on a totally depressing note.