Corelight-client

* Add support for Corelight Fleet Manager. New options:

--fleet
Specifies the network address of a Corelight Fleet Manager.

--uid
Specifies the UID of a Corelight Sensor managed through the
specified Corelight Fleet Manager.

--mfa
Specifies the 2FA verification code for authentication with
the specified Corelight Fleet Manager. Use '-' to ask the
user.

* Improve error handling.

* Make cache file handling more robust.

* Fix metadata caching.

* Do not perform UID check if certificate/hostname validation is disabled.

* Updating Twine configuration.

* Change `async` argument variable name to not be a Python keyword. (Ian Gallagher)

* Caching credentials separately for each device.

* Improve robustness against intermittent losses of connectivity
during device reconfiguration.