add a waiting reason when parallel updates has hit the max

[nicks]

Current Behavior

Tilt currently has a way to set the maximum number of parallel updates: https://docs.tilt.dev/api.html#api.update_settings

Tilt also has an API for determining why a resource is waiting: https://api.tilt.dev/interface/ui-resource-v1alpha1.html

But when a resource is pending because Tilt hit the maximum number of updates, there's no indication in the UI that this is the case!

Describe the Feature You Want

The waiting reason should say why the build hasn't started.

Additional context

The max update cap is handled separately from other kinds of "waiting" holds in tilt (because it depends on global state, rather than the state of a resource and its dependencies).

It's very important that "waiting" reasons don't "flap" back and forth between two reasons. For example, if you have 100 resources with the waiting reason "Number of updates maxed out", we don't want 99 resources to flip to a different reason, then flap to "Number of updates maxed out" when the new build is scheduled. This creates a lot of spurious updates that slow down the whole system.