Fix failing TCP server if much data is received

The Google protocol buffer documentation states that one has to reset
the size counter after each message in case multiple messages will be
read with one CodedInputStream.

cp. https://developers.google.com/protocol-buffers/docs/reference/java/com/google/protobuf/CodedInputStream.html#setSizeLimit(int)
Merge pull request #6 from jgosmann/fixes/tcpServerMsgSizeLimit

Fix failing TCP server if much data is received