Reduce the latency of streaming reads by re-using DN connections

The current libhdfspp implementation opens a new connection for each pread. For streaming reads (especially streaming short-buffer reads coming from the C API, and especially once we get SSL handshake overhead), our throughput will be dominated by the connection latency of reconnecting to the DataNodes.

The target use case is a multi-block file that is being sequentially streamed and processed by the client application, which consumes the data as it comes from the DN and throws it away. The data is read into moderately small buffers (~64k - ~1MB) owned by the consumer, and overall throughput is the critical metric.