This changes improve the binary upgrade behaviour using USR2:
- only one binary upgrade can happen at a time: the old arbiter needs to be
killed to promote the new arbiter.
- if a new arbiter is already spawned, until one is killed USR2 has no action
- if a new arbiter has been spawned, the unix socket won't be unlinked
- until the old arbiter have been killed the newly created pidfile has the name
<pidfile>.2 and the name Master.2 .
Note: there is no dialog between both arbiters to handle this features.
Instead they will supervise each others until one is killed. So isolation is
still guaranted.
fix#1267
This change add proper file locking to gunicorn. By default "gunicorn.lock" is created in the temporary directory when a unix socket is bound. In case someone want to fix the lock file path or use multiple gunicorn instance the "--lock-file" setting can be used to set the path of this file.
fix#1259
