Enable soft autocommit in default Solr config

Since upgrade from Solr 5.5 to Solr 6.6 (commit 6fe7359), hard
autocommits were still enabled to regularly persist the Solr index to
the file system, but new index entries were no more automatically made
available for use by the application (soft autocommit).
Therefore, YaCy features such as index statistics, that do not perform
an explicit commit (as recommended by Solr documentation) were no more
accurate.
Soft autocommit is now restored as a default, with a time period
expected to be sufficient for accuracy while adding only a reasonable
system load overhead.

Fixes issue #251

defaults/solr/solrconfig.xml

@@ -325,7 +325,7 @@
       -->
 
      <autoSoftCommit> 
-       <maxTime>${solr.autoSoftCommit.maxTime:-1}</maxTime> 
+       <maxTime>${solr.autoSoftCommit.maxTime:5000}</maxTime> 
      </autoSoftCommit>
 
     <!-- Update Related Event Listeners