Explore removing the walogs in tablet load in the same mutation that sets the location.

**Is your feature request related to a problem? Please describe.**

When a tablet server loads a tablet that has walogs assigned to it and the tablet has no data in those walogs it will write a mutation to delete the walogs from the tablets metadata. Shortly after this it will writer another mutation to set the tablet locaction. While working on #4873 analysis was done to see where time was spent when loading a tablet in this situation. After the changes in #4873, the time to write these two mutation was showing up more prominently. As the number of concurrent tablets in this situation increases two mutations when it could be one will double the load on the metadata table.

**Describe the solution you'd like**

It would be nice if these two mutations could be collapsed into a single mutation. The mutations are written in two different places currently. Deleting the walogs is done in the Tablet constructor. Updating the location is done in the AssignmentHandler class. Some refactoring will be needed to make this work.