Fork from origin mongodb, I want to add some features which not official open source, or not official support.

First, In-Memory Storage Engine

       official In-Memory storage engine manual
       https://docs.mongodb.com/manual/core/inmemory/#storage-inmemory

       below is the configure file example:

       processManagement:
	 fork: false
	 pidFilePath: ./mongod.pid
	
	net:
	 port: 27017
	 maxIncomingConnections: 200
	 unixDomainSocket:
	  pathPrefix: ./
	
	systemLog:
	 destination: file
	 logRotate: reopen
	 path: ./mongod.log
	 logAppend: true
	 verbosity: 5
	
	replication:
	 replSetName: mgset-name
	
	storage:
	 dbPath: ./data
	 engine: inMemory
	 inMemory:
	  engineConfig:
	   inMemorySizeGB: 1
	 mmapv1:
	  preallocDataFiles: true
	  nsSize: 16
	 wiredTiger:
	  engineConfig:
	   cacheSizeGB: 10
	
	security:
	 authorization: disabled
	 keyFile: ./keyfile