Currently the driver adds or removes hosts in response to SDAM events, and has some code to change server descriptions to unknown, but does not have a generic procedure to update description of a particular server. Such a procedure is required by rsghost handling which neither adds nor removes hosts but requires to checkifhasprimary after updating description. Without the description update rsghost handling is broken.

This issue will be partially fixed by 1492, this ticket is for auditing the remaining server description updates and refactoring the code to match pseudocode in the spec.

This ticket should also make it so that when a description is updated, the SDAM description updated event that gets sent has the correct old as well as new descriptions. (Audit the code for sending new description in both slots.)