mongodb: easy specifiers for safe writes and secondaries reads

[elucash]

Earlier, it was like a good idea to not provide any specific support for WriteConcern and ReadPreference.
In general, database defaults could be set on DB instance and repositories will just use that. Separated repositories of the same class may be configured to use different default options for WriteConcern and ReadPreference.

After some additional hands-on experience with other mongodb drivers, it is could be beneficial to add minimal support for per-operation overrides:

• safeWrite() for overriding write concern
• readSecondaries() for overriding read preference

Just as an example. insert, deleteAll, upsert etc — doesn't have configuration object, nowhere to stick this configurator. This will require some API overhaul, which is not very desirable. Not that we cannot change things, but it may complicate repository API.

[elucash]

currently there's no plans to improve this aspect of mongo support