fix: make `mongodb` dependency optional. Will throw error if package …

…not installed only if used with ReplicaSet class.

fix(MongoMemoryReplSet): use correct import for EventEmitter

closes typegoose#157

feat(ReplicaSet): set `electionTimeoutMillis` to 500ms instead of 10s…

… as default value

* electionTimeoutMillis default to 500ms

* All defaults in constructor

* Changed default to avoid accidental overriding

* Changed default to avoid accidental overriding

fix(build): disable flow definitions while be ready `flowgen` package