ephy-history-service: do not open a transaction in read-only mode

https://bugzilla.gnome.org/show_bug.cgi?id=735912
search-provider: don't live forever

Search providers are supposed to quit after running for a few seconds.

https://bugzilla.gnome.org/show_bug.cgi?id=735912
search-provider: tweak default timeout

As recommended by Giovanni in bug #736449