Implement input policies

[j-stephan]

The user currently has the option to set an input limit on stages. Once this limit is reached thread execution is blocked until the internal buffer has room for more objects.

There are circumstances in which this behaviour might be undesirable. ddrf should therefore offer the following three policies:

• block (default): The current behaviour. Block thread execution until there is room for another object.
• discard: If the buffer is full, discard the current object.
• overwrite: If the buffer is full, delete the oldest object and push the current object.