Increase socket read/write buffer size for better performance while uploading/downloading large attachments

[NFM-8]

Currently, the code uses a buffer of 4096 bytes when uploading or downloading attachments. I am not 100% sure if it is because of this, but attachment upload/download performance is a lot slower on phone than desktop when they are both connected to the same network.

I suggest that the buffer size is incresed to 32768 bytes. This is used in Telegram android client which does not suffer from such a slow file transfers as Signal (see https://github.com/DrKLO/Telegram/blob/4ebcbf61cd1ce04685c82f93b962d1094c5ffac2/TMessagesProj/src/main/java/org/telegram/messenger/ImageLoader.java, line 211).

[insideClaw]

@NFM-8
Should this be closed to match the proposed PR #57, which closed state implies this has been found not an issue?