Fix concurrent modification in UpdateManager.

If we add or remove components while an update check is ongoing
we used to throw a CME. This could happen either when the client
is first starting and in the process of adding components, or
if some listener decided to add/remove components as they were
being checked.

Change-Id: I200a3b4804bd7e815a4f053ca596b41f7c4c4c24
Fixes-issue: CLIENT-404
Reviewed-on: http://gerrit.dmdirc.com/2795
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>