Mojo should use flock to lock spec invocation

[Wes Mason]

It is possible to hit race conditions by invoking specs more than once, e.g from auto-rollout scripts, manually invoking, perhaps invoking different manifests, scripts, etc. within the same spec.

As a real example we had a case recently were a service was mysteriously being automatically downgraded to an older revision of a charm, because the upgrade script was simultaneously being invoked at the same time as a collect/repo phase which overwrote the contents of the charm payload prior to upgrade.

I think to avoid this the most workable solution would be an early flock invocation against a file in the workspace, which gets unlocked on exit (including on SIG* trap).