7.x 4.x sustainers performance improvements

 - Replaces strtotime('now') in a few places where being off by a second or two shouldn't matter.
 - Changes the by_member() call on the sustainer edit form to by_masterdid().
 - Updates the by_member() query to use the base table in the WHERE clause instead of the joined table.
 - Creates a count remaining donation query that doesn't use a LEFT JOIN or ORDER BY and does the count inside the query. This is used on the sustainer edit form and on the donation success hook implementation.