Bug #725402 - Checking sender for multiple images causes hang

This was introduced around commit ef355cd5, when the caching of
search results were dropped. The new function was also buggy, thus
this commit fixes even that.

Namely done changes:
- cache email address book search results for 5 minutes
- do not stop searching when any book fails to open (which can be
  due to temporary unreachable server, for example)
- search in local books first