Implement proper parallel flow of the rebalance execution

[whitehawk]

Implement proper parallel flow of the rebalance execution

Problem description:
Need to update rebalance execution flow in a way that it can support parallel
segment movement, and at the same time the flow must consider following
limitations:

• ggrebalance should save every move step and it's status in persistance
  storage so that failed steps may be retried, rollbacked or cancelled (rollback,
  retry or cancel of particular movement will be implemented later in a separate
  patch);
• switchover actions (primary to mirror, mirror to primary) will require user
  approval once we implement interactive mode (later in a separate patch);
• ggrebalance should consider the order of the planned movements in the
  primary-mirror swap scenario using 3rd intermediate, transitional host. It means
  that the executor can't swap the order of mirror and primary movements.

Therefore, this patch:

1. Adds an entity of RebalanceStep, that contain the state of execution together
   with the movement definition. List of such steps is now saved to the rebalance
   schema.
2. Updates the state machine of the rebalance execution. Now new states, where
   approval will be later requested from the user, are added. And the state machine
   can switch between segment processing and approval request as many times as
   required, till all steps are processed. Execution of the rebalance steps is
   performed in batches. Each batch is comprised from the same type of rebalance
   steps, without duplication of dbids.
3. Updates the code to use '--parallel' option to config
   'gpmovemirrors'/'gprecoverseg'.
4. Updates behave tests according to changes described above.

[hilltracer]

LGTM. Finally, behave tests are passed

1 feature passed, 0 failed, 26 skipped
32 scenarios passed, 0 failed, 808 skipped
1274 steps passed, 0 failed, 13181 skipped, 0 undefined
Took 550m51.963s
make: Leaving directory '/home/gpadmin/gpdb_src/gpMgmt'