Add credit-based flow control for stream consumers

Stream consumers had no explicit flow control. During replay, broker
delivery could outrun processing and overload downstream systems.

Use creditWhenHalfMessagesProcessed(initialCredits) when building the
stream consumer. Call context.processed() after each handled message so
credits replenish as processing advances.

Add stream.initialCredits to [stream] configuration. Its default is 1
and its minimum value is 1. Stream credits permit broker chunks, not
individual messages, and chunk sizes vary.

Queue consumers use prefetch 300, allowing up to 300 unacknowledged
deliveries. Stream consumers use chunk-based credits, so the values are
not directly comparable. The one-credit default keeps the initial stream
window to one chunk while replenishment follows processing progress.

Change-Id: I20f3050c8c6f52d98b82b0f86c5178ec77c53b95