MXS-1503: Enable limited session command history

Enabling the session command history but limiting it to a history of 50
commands allows reconnections for sessions that don't change the state too
often.

As pooled connections will exceed this limit quite fast, they are not able
to reconnect to servers once connections are lost. To solve this problem,
the session command history needs a compaction process that removes
redundant history.

Documentation/Release-Notes/MaxScale-2.3.0-Release-Notes.md

@@ -10,6 +10,24 @@ report at [Jira](https://jira.mariadb.org).
 
 ## Changed Features
 
+### Session Command History
+
+The _readwritesplit_ session command history is now enabled mby default but it
+is limited to a total of 50 distict session commands. This default allows most
+sessions to leverage the newly improved reconnection mechanism without having to
+explicitly enable the history. When the limit of 50 commands is exceeded, the
+session command history is disabled. This makes it possible to use pooled
+connections by default without having to explicitly disable the history (this
+was the case with pre-2.1 versions of MaxScale).
+
+The way that the history is stored has also changed. Instead of storing all
+session commands, each session only stores the first and last execution of each
+command. This way the history is compressed into a compact representation while
+still retaining the relative order of each command.
+
+To keep the old functionality, add `disable_sescmd_history=true` to the service
+definition.
+
 ## Dropped Features
 
 ## New Features