Asynchronous download/push/pull

[wonder-sk]

Adds support for asynchronous sync - i.e. to be able to start push/pull of a project, the download will run in a background thread and then when download is finished (it can be checked whether it has happened already), we do finalization of the job (which should not take long so it is in the main thread).

This should allow the GUI to 1. allow cancellation, 2. show progress.

The command line interface has been updated to have progress bar during download/push/pull and allows graceful cancellation.

The number of parallel request has been fixed to 4 so that we don't drain server resources too much. This could be modified later or made configurable. There is now a single code path unlike before with different code paths for parallel/non-parallel approach.

By the way, MerginProject object has been moved to a new file without any changes to the code.

TODO:

• remove old (non-async) implementations
• remove old (non-async) CLI support
• update tests not to test parallel/non-parallel code path
• add a test to check that code handles correctly pull with 1. deleted basefile, 2. corrupted basefile
• more documentation of the new code (?)

[wonder-sk]

Continued in #44