Implement input buffering

[satta]

This PR improves the socket input in FEVER by introducing a buffer for the input channel after parsing EVE from the socket.
The buffer is implemented using a variable-size buffered Go channel (--in-buffer) which will ensure that FEVER can keep up reading from the socket even if the processing pipeline or the forwarding receiver temporarily blocks.
If the buffer fills completely, we will optionally (--in-buffer-drop) rather drop parsed events from the buffer than cause writes to the input socket to block. This is all done to prevent Suricata from ever having to block its internal pipelines, leading to packet drops with an even wider impact.

We also send new metrics if the socket input is selected: input_buffer_length and input_buffer_dropped.