MongoBotContextManager storage logic

[achernin]

managers/mongo/src/main/kotlin/com/justai/jaicf/context/manager/mongo/MongoBotContextManager.kt

We have several problems when using MongoBotContextManager.

(1) it is not ready to support multiple simultaneous sessions with a one client -> sessions storage has to be a dictionary with the session id as a key
(2) ideally, i want to be able to define a strategy for BotContext._id , if BotRequest.clientid is not defined (in case of anonymous clients)