3 enhancements: add --no-dump, add --batch-size, reduce time processing large counts

Hi,

This PR adds 3 enhancements:

- adds a --no-dump option to mongo connector. 
- reduces time processing unnecessarily large counts (which is useful when the oplog is millions of documents large)
- adds a --batch-size <int> option to let users update the config file with current location in oplog incrementally while iterating through the oplog

Readme additions:

```
`--no-dump` tells mongo connector to begin scanning from the the top of the
 oplog rather than dump the entire namespace, if and only if the --oplog-ts
 file doesn't already have a record for the oplog.  Why might you want this?
  By default, if there is no timestamp for an oplog (ie if the --oplog-ts file
 is empty), mongo connector will dump the entire namespace.  You may want
 to disable that functionality.

`--batch-size` <int> lets you choose how many oplog documents to iterate
 through before updating the config file (pointed to by --oplog-ts) with the
 latest position read from the oplog.  By default, the oplog config isn't
 updated until we've read through the entire oplog.  You may want more
 frequent updates if the mongo connector process is at risk of falling behind
 the earliest timestamp in the oplog

```

This may break a few of tests.  I'd love feedback before I consider fixing tests.  (This way, I'll potentially save unnecessary work).