Refactor default abs sender (rubenlagus#272)

* refactor-default-abs-sender: move all returns to `try` scope

- it makes clear that return would not happen if some exception has been thrown

* refactor-default-abs-sender: extract `sendHttpPostRequest` method

* refactor-default-abs-sender: inline usage of `sendHttpPostRequest` in most occurrences

* refactor-default-abs-sender: extract creation of `HttpPost` with configuration to method

* refactor-default-abs-sender: rearrange code to ease up refactoring

* refactor-default-abs-sender: extract `sendMethodRequest` method

* refactor-default-abs-sender: move temporary file names to variables

* refactor-default-abs-sender: extract files downloading to separate methods

* refactor-default-abs-sender: extract creation of `Runnable` for async file download to method

* refactor-default-abs-sender: extract `assertParamNotNull` method

* refactor-default-abs-sender: rename `tryToDownloadToTemporaryFile` method