Update tailers for mongoriver 0.4 (tokumx support)

This commit is a companion to https://github.com/stripe/mongoriver/pull/11, which adds support for tokumx as mongodb alternative and revamps much of the tailer api.

r? @ebroder 

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For reference, notes on how I test this since this took a little bit of puzzling:

``` yaml
# Collection.yaml
test:
  test_collection:
    :columns:
    - id:
      :source: _id
      :type: TEXT
    - name:
      :source: nested.name
      :type: TEXT
    - number:
      :source: nested.int
      :type: INTEGER
    - title: TEXT
    :meta:
      :table: blog_posts
      :extra_props: true
```


Create an mongodb collection, insert a sample: `conn["test"]["test_collection"].insert({title: "title", nested: {name: "name1", int: 5}})`

Start mosql, see if it creates table, see if database contains the record:
``` ruby
sql = Sequel.connect("postgres://postgres@localhost")
tailer_db = sql[:mosql_tailers]
tailer_db.all

db2 = sql[:blog_posts]
db2.all
```

Try to update and insert more to see if new ones are added:
```ruby
conn["test"]["test_collection"].insert({title: "title2", nested: {name: "name1", int: 5}})
conn["test"]["test_collection"].update({title: "title2"}, {'$set' => {'nested.int' => 99}})
# restart mosql here, look to make sure no new fields are added and changes are picked up
db2.all
conn["test"]["test_collection"].update({title: "title2"}, {'$set' => {'nested.int' => 90}})
```

Testing if smooth updating works:
```
info = tailer_db.first
Oplog_collection.remove()
conn.drop_database("test")
sql.drop_table('blog_posts')
sql.drop_table('mosql_tailers')

create old style table and put an entry in it
sql.create_table?('mosql_tailers') do
  column :service,   'TEXT'
  column :timestamp, 'INTEGER'
  primary_key [:service]
end
new_info = info.dup
# new_info[:timestamp] = 1409262430
new_info.delete(:placeholder)
tailer_db = sql[:mosql_tailers]
tailer_db.insert(new_info)
# verify
tailer_db.all()
```

After restarting mosql, the blog_posts table should still be empty, but the tailers table should have an extra column filled with the new info. Adding inserts should also work.
Since we haven't shipped mongoriver yet, there's time to replace placeholder with anything. I don't have any strong feelings about this - if you feel that `position` is better and we should update mongoriver, I'll go ahead and do that.