Ability to control the commit behaviour of commitWithin

We need the ability to control the hard/soft commit behaviour of commitWithin parameter.

Since Solr 4.0, the commitWithin's performs a soft-commit which prevents slaves from picking up the changes in a master/slave configuration.

The behaviour I'm thinking is as follows:

1. By default, commitWithin will trigger soft commits.

2. But this behaviour can be overwritten in solrconfig.xml to allow commitWithin to perform hard commits, which will allow slaves to pick up the changes.

<commitWithin>
<forceHardCommit>true</forceHardCommit> <!-- default is false -->
</commitWithin>

Related to SOLR-4100