Queue cherry-picks based on branch comprehension

At present, cherry-pick bot performs all possible cherry-picks simultaneously. The upside of this is faster propagation of important fixes to all targeted branches, but it carries a risk of regression as well.

For example, A cherry-pick from dev to 6.x, and 6.x.0 could experience a regression if the .0 pick is successfully merged, but the fix to the main branch 6.x is not merged until after the next branching event. The fix would then be present in 6.x.0 and 6.x.2, but not 6.x.1.

Proposed new workflow would queue staging of cherry-picks such that changes are merged from newest to oldest feature branches.

• Picks should be staged by newest->oldest branch (6.5 first, then 6.2)
• When the feature branch pick merges, stage any release branch changes if they are ready and not already manually staged (6.5.0, 6.2.8)
• Have the bot monitor branches to make sure no gap appears while changes are merging. (6.5 fix to 6.5 and 6.5.0, but branching to 6.5.1 has occurred before 6.5 feature branch fix is merged)
• Automatically generate new picks to fill in gaps in release branches only.