Fix index pattern search error

When using the index name search, if you type quickly, then the search is not invoked after you have finished typing. This is because we had specified options to [`_.debounce`](https://lodash.com/docs#debounce) to execute the search function on the *leading* edge of the specified timeout, instead of the *trailing* edge. As a result, the index name search is invoked when you start typing, but not always after you have finished typing.  This change removes the overridden options, making the search function execute on the *trailing* edge of the specified timeout (i.e. after all input is received).

Closes #1899.