Use global timeout for network requests and script execution

Do not poll while waiting, simply wait for the signal.
A global and longer timeout of 60 seconds is now used for both
asynchronous network requests and script execution, which may
also include synchronous network requests. Properly set an
error message if the timeout expires.

This makes the engine also usable with very slow web servers
or internet connections (previously the timeout was around 15s).

Do not lock the mutex while the script gets evaluated in
ScriptJob::loadScript(). Access the request object of script
jobs only with locked mutex.