[http] Rework HTTP downloader

[http] Rework HTTP downloader
* Simplify code and split into separate routines to facilitate maintaining
* Make retry mechanism work on errors during actual download not only during connection establishment phase
* Retry on ECONNRESET and ETIMEDOUT during reading data from network
* Retry on content too short and various timeout errors
* Show error description on retry
* Closes #506, closes #809, closes #2849, closes #4240, closes #6023, closes #8625, closes #9483