Sftpretty

---------------------------
* added initial support for resuming existing transfers get() & put()
* added max_concurrent_prefetch_requests parameter to get() family
* added prefetch parameter to get() family

---------------------------
* added initial support for OpenSSH config-file support to CnOpts

---------------------------
* added drivedrop to bypass _adjust_cwd's lack of Windows drive support
* added file_size logic and default remotepath fallback to putfo
* moved Connection.compress to CnOpts.compress
* regression in put_d *again*
* removed SKIP_IF_CI from all but one test in response to the above
* switched to public key auth for all tests !¿macOS?¡
* switched from get_fingerprint() using md5 to helpers.hash using sha3_256
* test clean-up and major refactor

---------------------------
* fix regression in put_d

---------------------------
* allow CnOpts knownhost to be set to None directly
* standardize on using is for None checks

---------------------------
* added log_level to connection options
* added compression security option for Transport
* code optimizations in _start_transport() and _set_authentication()
* moved compression on/off switch to Connection object
* sprinkled debug messaging throughout
* switched to using native logging module instead of paramiko util