Maintenance: reduce default session director names

[crowecawcaw]

Description

Some Windows DCCs do not support long paths, so file path support is limited to 260 characters. Deadline/OJD uses a large portion of those for paths that contain long IDs including session directories.

Solution

Use 8 random characters for the session directory, not something that contains the session ID.

See OpenJobDescription/openjd-sessions-for-python#348