Use per-thread QDBusConnections

This moves DBusConnectionPool from Nepomuk search code to Akonadi and makes
use of it in search infrastructure and couple other classes that interact
with DBus from non-main thread.

QDBusConnection is not thread-safe in Qt 4, so we need to workaround it by
having a connection for each thread. Qt 5 should be OK, so we can remove this
in Frameworks.

This should fix random crashes I've been seeing when SearchTaskManager::addTask()
was called from multiple threads simultaneously.