Simplify the logic to create the OplogSlurper threads

In both the sharded and the non-sharded case we're actually running the same code, with the minute
difference of avoiding to get a specific MongoClient. This however isn't needed, as those clients
are cached anyways.

Remove the condition on mongos, and just create threads for each shard.
Stop implementing Runnable in CollectionSlurper

The CollectionSlurper class is really a helper, and not something that runs in a separate thread. Removing
the Runnable allows us to give a reasonable name to the method that does the initial import, and later on
also allows to provide parameters to the methods rather than to the class.
Move the 'timestamp' parameter from the constructor to the method(s)

This makes it more obvious that this timestamp is a detail that is only relevant to the
way the implementation works.

While there: document the parameter.
Start importing from the last timestamp when a river is stopped and started again

The river remembers the timestamp it last imported, but this information wasn't used
when the tailer threads start up. If a river was stopped for a while (for example because
it failed and was restarted) data between the last import time and the current shard
oplog time was never processed.

Note that the test does some aggressive reflection to restart the river, this would become
obsolete with #407 merged.
Remove a now-unused method
Remove unused import
Remove the reflection code needed to restart the river
Merge pull request #433 from Collaborne/river-initial-skip-initial-import

River silently loses documents on restarts