XDS snapshot updates are not correctly delivered after a first retry when certain conditions are met.

[dhiaayachi]

Overview of the Issue

This is sequel to #9689.

When an XDS snapshot is received and we set a timer to update after a coalesceTimer in here, if the 1st condition from #9689 is met and we end up in the default case of the select in here the coalesceTimer is never reset and snapshot updates end up never triggered again until the service is recreated because coalesceTimer will never be nil and would prevent sendCh from ever be signaled.

This issue would happen more frequently in overloaded systems, with a high number of services and a high number of XDS updates which will more likely trigger the 1st condition from #9689.