remote: add session feature in labgrid-client

[luke-hackwell]

Description
Adds a --session option to the labgrid-client monitor, reserve and acquire commands.
Enables the user to setup a monitor session. Reservations and place acquisitions which are registered to the same monitor sessions will be released if the monitor session ends.

This is useful for reservations and places acquired under a CI job which may be killed without warning.

Example usage:

labgrid-client monitor --session $$ &
MONITOR_PID="$!"
eval $(labgrid-client reserve  --session $$ --shell tag=value)
labgrid-client -p + lock --session $$
kill "${MONITOR_PID}"

Sessions are unique for the same IP address and session name.

Checklist

• Documentation for the feature
• Tests for the feature

• The arguments and description in doc/configuration.rst have been updated

• Add a section on how to use the feature to doc/usage.rst

• Add a section on how to use the feature to doc/development.rst

• PR has been tested

• Man pages have been regenerated