OF-1487: Monitoring plugin: write archived data to the database more often.

Prior to this commit, archived data was kept in a buffer, which was flushed to the database once per minute.
It is undesirable to have the database 'lag behind' for such an extend period of time.

This commit introduces near-instantanious flushes. Data that's put in a buffer is flushed immediately, unless
more data is available instantly.

src/plugins/monitoring/changelog.html

@@ -47,6 +47,7 @@ Monitoring Plugin Changelog
 <p><b>1.6.0</b> -- Feb 16, 2018</p>
 <ul>
     <li>[<a href='https://issues.igniterealtime.org/browse/OF-1486'>OF-1486</a>] - MAM RSM queries for MUC should allow for 'backwards-paging'.</li>
+    <li>[<a href='https://issues.igniterealtime.org/browse/OF-1487'>OF-1487</a>] - Archived messages should become available in the database instantly.</li>
 </ul>
 
 <p><b>1.5.9</b> -- Feb 13, 2018</p>