Dvc: `dvc push --run-cache` is incredibly slow on "ssh" remote

[peper0]

Bug Report

A command dvc push --run-cache takes very long time (more than hour in my case) if there are a lot files in dvc_cache/runs. The reason is making a new ssh connection for each file to check if it exist on the remote.

Please provide information about your setup

Output of dvc version:

$ dvc version
1.8.4+21a6be

Additional Information (if any):

$ dvc push --run-cache --verbose
2020-10-16 14:36:14,695 DEBUG: Check for update is enabled.
2020-10-16 14:36:14,697 DEBUG: fetched: [(3,)]                        
2020-10-16 14:36:14,818 DEBUG: Establishing ssh connection with 'xxx' through port '22' as user 'dvc'
2020-10-16 14:36:15,457 DEBUG: Establishing ssh connection with 'xxx' through port '22' as user 'dvc'
2020-10-16 14:36:16,070 DEBUG: Establishing ssh connection with 'xxx' through port '22' as user 'dvc'

and so on, hundreds of time.

[peper0]

It is caused by dropping ssh connection out of the pool in get_connection in dvc/tree/pool.py since the connection throws a GeneratorExit exception in generator SSHTree.walk_files if the generator is closed prematurely, which is the case in dvc/stage/cache.py:210 in the statement first(to_remote.walk_files(key)).

See my PR.