Allow MongoDB ReadPreference configuration

[jimlambie]

API needs a way to configure the ReadPreference when connecting to a replica set, as per this page in the docs: https://docs.mongodb.com/v3.0/core/read-preference/

[jimlambie]

@fra967 can you please confirm this covers what you require for readPreference?

https://docs.mongodb.com/manual/reference/read-preference/

[fra967]

We will probably want to set it to secondaryPreferred in order to fall back to the primary.
I believe we also need to issue a rs.slaveOk() before sending the query (at least, that's what needs to be done from the shell, not sure if the drive would do that automatically)

[jimlambie]

@fra967 I believe that happens internally, as seen by this getter on the Db class:

// slaveOk specified
Object.defineProperty(Db.prototype, 'slaveOk', {
  enumerable:true,
  get: function() {
    if(this.s.options.readPreference != null
      && (this.s.options.readPreference != 'primary' || this.s.options.readPreference.mode != 'primary')) {
      return true;
    }
    return false;
  }
});

[fra967]

cool, so I guess we just need to test it :)

[jimlambie]

@fra967 do you have a preference for the default setting? or should secondaryPreferred be default?

[fra967]

yes @jimlambie please let's use secondaryPreferred as default value

[jimlambie]

Rolled into #171