conf/output: Buffering related config settings

This commit adds 2 EVE output buffering settings
- buffer-size value which specifies the amount of buffering, if any,
  for regular/file output types.
- flush-interval Specifies the cadence at which Suricata will direct
  detect threads to flush EVE output.

Issue: 3449

suricata.yaml.in

@@ -101,6 +101,10 @@ outputs:
       # Enable for multi-threaded eve.json output; output files are amended with
       # an identifier, e.g., eve.9.json
       #threaded: false
+      # Specify the amount of buffering, in bytes, for
+      # this output type. The default value 0 means "no
+      # buffering".
+      buffer-size: 0
       #prefix: "@cee: " # prefix to prepend to each log entry
       # the following are valid when type: syslog above
       #identity: "suricata"
@@ -571,6 +575,16 @@ outputs:
 # Logging configuration.  This is not about logging IDS alerts/events, but
 # output about what Suricata is doing, like startup messages, errors, etc.
 logging:
+  # The flush-interval governs how often Suricata will instruct the detection
+  # threads to flush their EVE output. Specify the value in seconds [1-60]
+  # and Suricata will initiate EVE log output flushes at that interval. A value
+  # of 0 means no EVE log output flushes are initiated. When the EVE output
+  # buffer-size value is non-zero, some EVE output that was written may remain
+  # buffered. The flush-interval governs how much buffered data exists.
+  #
+  # The default value is: 0 (never instruct detection threads to flush output)
+  #flush-interval: 0
+
   # The default log level: can be overridden in an output section.
   # Note that debug level logging will only be emitted if Suricata was
   # compiled with the --enable-debug configure option.