[feature/server] Create Basic Implementation for IMessageQueue

[SubverseIM]

The implementation contained within Subverse.Server requires a persistent, unordered message queue via IMessageQueue that has the following requirements. It:

• Can be accessed concurrently by different threads within the server process
• Can be queried and filtered by key, that being the intended recipient of the message at the time that it was queued
• Must be efficient:
  • Time to dequeue a message by key should be O(log n)
  • Time to dequeue a message without a specific key should be O(1)

Additional requirements:

• Backing store must be encrypted
• Backing store must be persistent between server process & machine restarts

[IsaMorphic]

Consider using LiteDB for the backing store, it plays well with the goal of using BSON as a network message protocol.