Use a unique machine identifier for generating pool ids

[albrow]

Currently, a rebooted machine would get a new pool id. If it was in the middle of executing any jobs when it rebooted, those jobs would be stale. In order to detect that the new pool will attempt to ping the old pool, then when it doesn't get a response it would re-queue the stale jobs. This is a little more work than what is necessary.

If the same machine always gets the same pool id, it would allow for a machine to quickly clean up after itself in the event that it is rebooted. On initialization, it could check for any jobs in the executing state with it's own pool id. We wouldn't need to try to ping the old pool to determine that those jobs are stale.

[albrow]

This is now fixed in the develop branch (see 416b0f2) and will be merged into master for the next release.