Use psqueue library

This change will be used to implement the scheduling of the data
collectors.

Signed-off-by: Aaron Karper <akarper@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>
Add design goal for multiple disk templates per instance

The original design document for the disks didn't mentioned that there
is also the assumption that all disks for a single instance are of the
same template.

Signed-off-by: Aaron Karper <akarper@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>
Add design doc: sync rate throttling

Lays out a functionality that sets the drbd sync rate depending on the
number of devices syncing. Later versions would also adapt to
inter-group syncs and handle flow more effectively.

Relates to issue 935.

Signed-off-by: Aaron Karper <akarper@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>