Fix server address settings change so it properly logs out the current
account, if any.

The probe for the new account was inadvertantly logging out the current
account, partially, on failure of the login probe (due to false
credentials), disrupting the provision to fully log out on successful
server address change. To correct for that, prevent any login status
changes if an alternate host address is being probed.

For consistency, we also prevent follow-through on login success, even
if the credentials succeed.

Additionally, remove some obsolete residual comments, including one
concerning enforcing case sensitivity.
Merge pull request #430 from SpiderOak/issue-428-fix-server-address-logout

Issue #428 - Fix server address settings change so it properly logs out the current account, if any.