Lock access to reading the generation.
Lock access to reading the generation.

Golang will panic if a map is concurrently read and written.
We guard the map write (L303) but we also need to protect when we read from this map.

Switch to use an RWMutex so that we can concurrently read.

GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid