Add support for replica set

I added support for replica set with a minimal API modification:
  - on Db class
      - Db.poll constructor added on Db class
      - isMaster method added
      - serverConfig attribute removed
      - connection attribute renamed _masterConnection and keep it private

ConnectionManager class is in charge of managing the replica set connection (even if no replica set defined).

Regarding tests, I started with a new collection of tests in replica_tests.dart file, but I would like to modify legacy tests to be processed with standalone and replica set configuration : in other words, variable db could be a parameter of each tests and processed for both db configuration. What do you think about this ?

A library test helper (named mongo_action.dart) is there to facilitate test procedure.

Before adding a connections polling and reconnection in new ConnectionManager class, I would wait your remarks.