Paste - upgrade scheme to support queries by authorPHID

Test Plan: ran upgrade_schema.php on my local install. re-examined SQL query
"explain select * from phabricator_pastebin.pastebin_paste where authorPHID =
"PHID-USER-xee4ju2teq7mflitwfcs" ORDER BY id DESC;" on my local install and
noted fewer rows scanned, index used, etc

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1218