[FTS] Limit number of returned results by default

Currently FTS makes it too easy for users to ask for very large numbers of stored documents, which can lead to the system crashing. For example, some of the random query test cases are expecting 2gb http responses.

To avoid this problem, add a default "limit_results" property value that is set to a more reasonable number of results.

Note that the most common search use cases rely on pagination to limit the actually returned values to a small subset of the total matching documents.

In use cases where a list of all document IDs matching a term is returned (for example, when combining FTS results with N1QL results), only the IDs are needed, not the stored fields. Implementation of limit_results property should not rule out such use cases.