Prevents timeouts during long write operations on the master side

Adds a writeComplete() handler on the Server.ServerHandler to update
 the corresponding transaction's timestamp
Properly handles exceptions for finishing channels that do not really
 have corresponding transactions
Introduce and use special class for transaction missing on master

Instead of catching an IllegalStateException, we use a specific
 TransactionNotPresentOnMasterException when the resume of a tx for
 finishing it is not present. That makes the corresponding catch clause
 more specific, avoiding swallowing a potential error.

Also makes sure that writeComplete() in Server.ServerHandler forwards the
 call down the pipeline
Merge pull request #888 from digitalstain/transaction-timeout-fix

Prevents timeouts during long write operations on the master side