This PR adds 2 defensive mechanisms for shell snapshotting:
* Filter out invalid env variables (containing `-` for example) without
dropping the whole snapshot
* Validate the snapshot before considering it as valid by running a mock
command with a shell snapshot
Clean all shell snapshot files corresponding to sessions that have not
been updated in 7 days
Those files should never leak. The only known cases were it can leak are
during non graceful interrupt of the process (`kill -9, `panic`, OS
crash, ...)