Mismatch navigation can hang the main thread for a while

Since UGENE-5272 is done, one can navigate through mismatches. The comparison between consensus and reference sequence is in the main thread and it depends on the consensus cache, which is calculated on demand. So, the mismatch search causes consensus cache updating on whole alignment, which hangs GUI for a couple of seconds.
Move mismatch search or consensus cache updating to a separate thread.