Fixed PHP-818: Add deprecation notices to usage of Mongo.

For the tests, I also swapped to using "MongoClient" by default and looked at
individual tests at what they were testing to decide whether to go with
"old_mongo" specifically or mongo/new_mongo() or equivalents. I guess some
choices could be debated though.

In one case (tests/generic/bug00635.phpt) I changed the expected output (beyond
deprecation notices), but this test was testing something unrelated to w=0/w=1.