Custom LuceneFieldSorter created by plugins don't get flushed on plugin restart

If a plugin developer registers an own CustomFieldSearcher and implements getSorter, the returned LuceneFieldSorter is being cached by Lucene. This cache isn't cleared when the plugin reloads, which causes a memory leak.

In addition to that, if the custom field is part of the issue navigator configuration, the old sorter from the dead classloader is used and can cause errors similar to this, breaking the issue nav:

An error occurred whilst rendering this message. Please contact the administrators, and inform them of this bug. Details: ------- org.apache.velocity.exception.MethodInvocationException: Invocation of method 'getHtml' in class com.atlassian.jira.issue.fields.layout.column.ColumnLayoutItemImpl