Refactor Velox Writer to Use New Flush Policy Contract (facebookincubator#242)

Summary:

This should be a no-op since no chunking flush policy is currently being used in Prod. but we make three changes in this dif:
1. `writeChunk` now returns a boolean to indicate whether any stream was successfully chunked
2. The previous raw size of the encoded stripe data in the writer context is now stored in the Writer context
3. We update and pass down the memory stats needed by the new flush policy contract

TODO: We will be introducing two more VeloxWriter changes in the next diffs in this stack to:
1. Support per stream chunking instead of always chunking all eligible streams
2. Support breaking down large stream into multiple smaller chunks

Differential Revision: D81545433

Author: macvincent

dwio/nimble/common/MetricsLogger.h

@@ -74,6 +74,7 @@ struct FileCloseMetrics {
 };
 
 enum class LogOperation {
+  ChunkWrite,
   StripeLoad,
   StripeFlush,
   FileClose,