Based on my experimental diagnosis it apperas the photoslibraryd daemon is causing some interaction with the system after a period of user inactivity on 2015 Macs. There have been many reports on variuos forms where 2015 MacBooks, Airs and iMacs are experiencing this issue. I've been able to stop the freezing from occuring on my 2015 Macbook Pro by moving aside the com.apple.photolibraryd.plist located the the /System/Library/LaunchAgents folder located on the boot volume. The moving aside of the plist file needs to be done under Recovery Mode using the terminal app. To accomplish the move I created a new folder called "LaunchAgents (Disabled)" in the /System/Library folder and used the "mv" command to move the plist file from the LaunchAgents folder to the "LaunchAgents (Disabled)" folder. I then rebooted, logged in and observed the system did not freeze while runnning the screensaver for more than 24 hours time. To verify my conclusion and repeatability I moved back the photoslibraryd daemon plist file to the LaunchAgents folder once again using Recovery Mode and rebooted to allow the daemon to start again at boot. After logging in and letting my screensaver start I observed a hard system freeze once again on my MacBook Pro 2015 after a few minutes. I'm not interesting in using the photos app at this time so disabling the daemon is of no consequence for me. I would rather have a stable system I can do my testing on. This seems to be a weird issue to say the lease ... Let's hope it's resolved in the next beta!
Created feedback FB6917382 documenting the findings and workaround.
Note: This workaround was originally posted under the "2015 MacBook Pro freezing after inactivity" posting. It was brought out as a separate post for higher visibility so others experiencing this issue can benefit.
