Add pool of SFTPClient sessions

[abichinger]

This PR should make fs.sshfs thread safe. #17

Changes:

• Add SFTPClientPool - a pool of SFTPClient sessions to share across threads
• New SSHFS arguments

  max_connections (int): Maximum number of concurrent SFTPClient sessions
          (defaults to 4)
  conn_timeout (float): The maximum time to wait for a free session. 
      Defaults to the value of ``timeout``.
  

• Use docker image linuxserver/openssh-server to run tests.
  I got the following error when I tried to pull sjourdan/alpine-sshd:

  [DEPRECATION NOTICE] Docker Image Format v1 and Docker Image manifest version 2, schema 1 support is disabled by default and will be removed in an upcoming release. Suggest the author of docker.io/sjourdan/alpine-sshd:latest to upgrade the image to the OCI Format or Docker Image manifest v2, schema 2. More information at https://docs.docker.com/go/deprecated-image-specs/