Fix a few exceptions in the oplog observe driver

First exception: _runQuery didn't check to see if it was stopped after
running the query, which could lead to this harmless error:

Exception in defer callback: TypeError: Cannot call method 'clear' of null
    at _.extend._publishNewResults (packages/mongo-livedata/oplog_observe_driver.js:749)
    at _.extend._runQuery (packages/mongo-livedata/oplog_observe_driver.js:657)
    at packages/mongo-livedata/oplog_observe_driver.js:615
    at _.extend.withValue (packages/meteor/dynamics_nodejs.js:56)
    at packages/meteor/timers.js:6
    at runWithEnvironment (packages/meteor/dynamics_nodejs.js:108)

Second exception: _fetchModifiedDocuments thought that it should only be
in FETCHING in a certain case, but QUERYING is also OK. This is also
harmless since the correct behavior is to end the deferred routine.

Exception in defer callback: Error: phase in fetchModifiedDocuments: QUERYING
    at packages/mongo-livedata/oplog_observe_driver.js:435
    at packages/mongo-livedata/oplog_observe_driver.js:16
    at _.extend.withValue (packages/meteor/dynamics_nodejs.js:56)
    at packages/meteor/timers.js:6
    at runWithEnvironment (packages/meteor/dynamics_nodejs.js:108)
Add a lot of _noYieldsAllowed to oplog driver

Makes it easier to reason about where yields are (and ensure that
_stopped is checked after all yielding calls)