Removed last traces of Redis from webapp and added better error handling

(in case the broker is down)
Implemented timeouts for transactions

The transaction map is scanned by default every ten seconds.
Transactions that are still in the map after a second scan are handled
by replying to the appropriate sender (broker/webapp) and removed.

The transaction_min_timeout determines how long a transaction should be
left in the map in the worst case; i.e. transaction will stay there
for ~15 seconds on average.

           Transaction is "incremented" at those points and removed the
Check     second time:↓
↓          ↓          ↓
|~~~~~~~~~~|~~~~~~~~~~|~~~~~~~~~~|~~~~~~~~~~|~~~~~~~~~~|~~~> (time)
   ↑
   Transaction is inserted