Batch HTTP/2 upload body flushes

[pavel-ptashyts]

Summary

• batch HTTP/2 upload DATA frame flushes while the stream remains writable
• flush only when the body pump yields for source suspension/backpressure or writes the terminal DATA frame
• prevent synchronous writability callbacks from re-entering the pump
• keep the pump parked while the terminal DATA write is pending, preventing duplicate endStream frames
• cover multi-chunk batching, suspension, backpressure resume, re-entry, and cleanup timing in Http2BodyWriterTest

Rationale

The HTTP/2 upload body pump previously flushed after every intermediate DATA chunk. That kept writes moving, but it also forced one flush per chunk even when the stream child channel stayed writable. Batching until a yield point preserves bounded in-flight data while reducing flush overhead for multi-chunk uploads.

Netty may synchronously fire channelWritabilityChanged from flush(). Explicit pumping and terminal-write states prevent that callback from mutating pump state recursively or emitting a second terminal frame before the first write completes.

Validation

• ./mvnw -pl client -Dtest='org.asynchttpclient.netty.request.body.Http2BodyWriterTest' test (3 tests)
• ./mvnw -pl client -DskipTests compile
• ./mvnw -pl client -Dtest=Http2StreamingBodyFlowControlTest test
• ./mvnw -pl client -Dtest=BasicHttp2Test test

Attribution

Codex on behalf of Pavel Ptashyts