CRM-15060 - don't load entire contacts table when reserving for survey.

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* CRM-15060: reserving survey respondents causes wait while entire database query is executed
  https://issues.civicrm.org/jira/browse/CRM-15060
Merge pull request #3764 from jmcclelland/reserve-respondents-force

CRM-15060 - don't load entire contacts table when reserving for survey.