Adding block kind to protocol for future streaming of a tree. am: 85390048

static constexpr int kBlockSize = 4096;

static constexpr int kCompressedSizeMax = kBlockSize * 0.95;

static constexpr short kCompressionNone = 0;

static constexpr short kCompressionLZ4 = 1;

static constexpr int8_t kTypeData = 0;

static constexpr int8_t kCompressionNone = 0;

static constexpr int8_t kCompressionLZ4 = 1;

static constexpr int kCompressBound = std::max(kBlockSize, LZ4_COMPRESSBOUND(kBlockSize));

static constexpr auto kReadBufferSize = 128 * 1024;

static constexpr int kPollTimeoutMillis = 300000;  // 5 minutes

using FileId = int16_t;

using BlockIdx = int32_t;

using NumBlocks = int32_t;

using CompressionType = int16_t;

using BlockType = int8_t;

using CompressionType = int8_t;

using RequestType = int16_t;

using ChunkHeader = int32_t;

using MagicType = uint32_t;

// Placed before actual data bytes of each block

struct ResponseHeader {

    FileId file_id;                    // 2 bytes

    CompressionType compression_type;  // 2 bytes

    BlockType block_type;              // 1 byte

    CompressionType compression_type;  // 1 byte

    BlockIdx block_idx;                // 4 bytes

    BlockSize block_size;              // 2 bytes

} __attribute__((packed));

        ++compressed_;

        blockSize = compressedSize;

        header = reinterpret_cast<ResponseHeader*>(data);

        header->compression_type = toBigEndian(kCompressionLZ4);

        header->compression_type = kCompressionLZ4;

    } else {

        ++uncompressed_;

        blockSize = bytesRead;

        header = reinterpret_cast<ResponseHeader*>(raw);

        header->compression_type = toBigEndian(kCompressionNone);

        header->compression_type = kCompressionNone;

    }

    header->block_type = kTypeData;

    header->file_id = toBigEndian(fileId);

    header->block_size = toBigEndian(blockSize);

    header->block_idx = toBigEndian(blockIdx);

bool IncrementalServer::SendDone() {

    ResponseHeader header;

    header.file_id = -1;

    header.block_type = 0;

    header.compression_type = 0;

    header.block_idx = 0;

    header.block_size = 0;