Force commands to use primary read preference

[jmikola]

While investigating #180, I came across some undocumented behavior in the legacy mongo extension that enforces a primary read preference in php_mongo_runcommand(). The php_mongo_command_supports_rp() helper only respects the MongoDB instance's read preference for teh following commands:

• count
• group
• dbStats
• geoNear
• distinct
• aggregate
• collStats
• geoSearch
• mapreduce and mapReduce (case-sensitive reasons)
• parallelCollectionScan

Note that aggregate does not check for write variations that would output to a collection; however, mapReduce does check for { out: { inline: 1}}. For that reason, a fix for Doctrine MongoDB may not be warranted, since its query builder does not support aggregate. It's possible that it may require a fix for the aggregation builder, though.