Flush based on buffer size rather than time

this includes:
- Add Accumulator to the Start() function of service inputs
- For message consumer plugins, use the Accumulator to constantly add
  metrics and make Gather a dummy function
- rework unit tests to match this new behavior.
- make "flush_buffer_when_full" a config option that defaults to true

closes #666

etc/telegraf.conf

@@ -16,23 +16,37 @@
 
 # Configuration for telegraf agent
 [agent]
-  # Default data collection interval for all plugins
+  ### Default data collection interval for all inputs
   interval = "10s"
-  # Rounds collection interval to 'interval'
-  # ie, if interval="10s" then always collect on :00, :10, :20, etc.
+  ### Rounds collection interval to 'interval'
+  ### ie, if interval="10s" then always collect on :00, :10, :20, etc.
   round_interval = true
 
-  # Default data flushing interval for all outputs. You should not set this below
-  # interval. Maximum flush_interval will be flush_interval + flush_jitter
+  ### Telegraf will cache metric_buffer_limit metrics for each output, and will
+  ### flush this buffer on a successful write.
+  metric_buffer_limit = 10000
+  ### Flush the buffer whenever full, regardless of flush_interval.
+  flush_buffer_when_full = true
+
+  ### Collection jitter is used to jitter the collection by a random amount.
+  ### Each plugin will sleep for a random time within jitter before collecting.
+  ### This can be used to avoid many plugins querying things like sysfs at the
+  ### same time, which can have a measurable effect on the system.
+  collection_jitter = "0s"
+
+  ### Default flushing interval for all outputs. You shouldn't set this below
+  ### interval. Maximum flush_interval will be flush_interval + flush_jitter
   flush_interval = "10s"
-  # Jitter the flush interval by a random amount. This is primarily to avoid
-  # large write spikes for users running a large number of telegraf instances.
-  # ie, a jitter of 5s and interval 10s means flushes will happen every 10-15s
+  ### Jitter the flush interval by a random amount. This is primarily to avoid
+  ### large write spikes for users running a large number of telegraf instances.
+  ### ie, a jitter of 5s and interval 10s means flushes will happen every 10-15s
   flush_jitter = "0s"
 
-  # Run telegraf in debug mode
+  ### Run telegraf in debug mode
   debug = false
-  # Override default hostname, if empty use os.Hostname()
+  ### Run telegraf in quiet mode
+  quiet = false
+  ### Override default hostname, if empty use os.Hostname()
   hostname = ""