kgo: add backpressure to batchPromises ring buffer

[twmb]

The batchPromises ring buffer in the producer could grow unboundedly.
When records fail faster than finishPromises() can process them (e.g.,
spin-loop producing failing records), the ring would grow without bound,
potentially causing OOM within the client.

This commit rewrites the ring as a single dynamically-sized circular
buffer with optional max length blocking:

• Buffer starts at capacity 8, grows by doubling when full
• Shrinks back to capacity 8 when at 4 or fewer elements
• Adds initMaxLen() to block pushes when at capacity
• Initializes batchPromises with max(maxBufferedRecords, 8192) limit

The blocking pushes back-pressure to the application rather than OOMing
within the client.

Closes #1194