Cleanup transfer manager

[douglas-raillard-arm]

Cleanup and simplify transfer manager infrastructure to make it more reliable.

WARNING: Not tested on SSH connection yet. Tested on android along https://github.com/ARM-software/devlib/pull/618

[mrkajetanp]

With the above changes the PR makes the PerfettoCollector work fine without specifying an explicit pull timeout so looks like it works :)

[douglas-raillard-arm]

@mrkajetanp I updated the PR with your review, thanks for testing it

[mrkajetanp]

We should be good to go then! I'll update the Collector PR to drop the timeout in that case

[douglas-raillard-arm]

PR updated with both typos fixed

[douglas-raillard-arm]

PR updated:

• TransferHandleBase defers the logger to its self.manager
• transfer_mgr => transfer_manager
• Fixed callback of SFTP transfer
• Fixed time.monotonic() imports

Thanks a lot for the testing

[douglas-raillard-arm]

PR updated with:

• fixed xfer percent formula for SFTP

[douglas-raillard-arm]

PR updated with s/mgr/manager, and rebased on master (there was a conflict so we might want to give it a last spin before merging)