While testing I found that over slow connection it was possible that some parts of stanzas where not sent properly due to fact that network buffer was full. I added additional write queue to deal with case.

To properly handle multithreading (needed for write queue, as data may be written to queue from UI thread, while it may be written to socket using other thread), I created separate serial task queue for each connection, which will be responsible for serialization, compression of data, decompression and XML parsing. This part was planned as part of task #4245, however it was required to fix this issue.

Changed due date due to found another issue in application.