fix(block-builder): return from Process call early if max offset is reached

[ashwanthgoli]

What this PR does / why we need it:
Process method is incorrectly polling forever even when the Max offset for the job is already processed.

Which issue(s) this PR fixes:
Fixes #

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• Documentation added
• Tests updated
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[owen-d]

small bit, but lgtm

[owen-d]

I think this needs a break instead so we can still send partial results through the channel. Otherwise, we'll return before sending any AppendInputs to the channel. This can cause problems if half the batch we're looking at is in the offset bounds we want and half is greater.