** Manages connections to a MongoDB instance.
** @see `ConnectionManagerPooled`
const mixin ConnectionManager {
    ** Basic details of where this 'ConnectionManager' connects to, for debugging purposes.
    ** When connecting to replica sets, this should indicate the primary. 
    ** It *should not* contain any user credentials and *should* be safe to log. 
    abstract Uri? mongoUrl()
    ** The default write concern that all write operations use if none supplied.
    ** Defaults to '["w": 1, "wtimeout": 0, "j": false]'
    **  - write operations are acknowledged,
    **  - write operations never time out,
    **  - write operations need not be committed to the journal.
    abstract Str:Obj? writeConcern()

    ** The name of the database users are authenticated against.
    ** If null, users are authenticated against the current database.
    abstract Str? authSource()
    ** Makes a connection available to the given function.
    ** What ever is returned from the func is returned from the method.
    abstract Obj? leaseConnection(|Connection->Obj?| c)
    // Can't return 'This' because of Plastic Proxies.
    ** Does what ever the 'ConnectionManager' needs to do to initialise itself.
    ** Often this would be create database connections or other network related activity that it 
    ** may not wish to do inside a ctor.
    abstract ConnectionManager startup()
    // Can't return 'This' because of Plastic Proxies.
    ** Closes all MongoDB connections.
    abstract ConnectionManager shutdown()