high cpu usage

[moham96]

some problem related mostly to the streaming feature:

1- This line and the pipeline that it is calling are causing degradation in performance because the functions involved in pushing the bytes like _calculateLastDownloadedByte are time consuming.

dtorrent_task/lib/src/file/download_file.dart

Line 213 in c87eb30

_bytesRequestController.add(null);

2- currently streaming is not very responsive because the read request has to wait before it is processed and returned

3- non flushed pieces are not read, because we are using 2 different file handles, one for writing and one for reading, and the reading handle will not be able to read non flushed bytes

4- since we are using different handles for read and write, we should have two streams to handle read and write requests