Reduce Maniphest fulltext limit from PHP_INT_MAX to 10k

Summary:
PHP_INT_MAX is rejected by ElasticSearch since it's outside of the representable integer range (see: https://gist.github.com/JustinTulloss/c4ac0e1c93d6d1e91744).

Just use 10K, as matching more than 10K results probably isn't useful to anyone.

Test Plan: Confirmed this fixes the issue in IRC. Ran a fulltext search.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7097